Tag Archives: Social Networks

Innovations in Graph Representation Learning



Relational data representing relationships between entities is ubiquitous on the Web (e.g., online social networks) and in the physical world (e.g., in protein interaction networks). Such data can be represented as a graph with nodes (e.g., users, proteins), and edges connecting them (e.g., friendship relations, protein interactions). Given the widespread prevalence of graphs, graph analysis plays a fundamental role in machine learning, with applications in clustering, link prediction, privacy, and others. To apply machine learning methods to graphs (e.g., predicting new friendships, or discovering unknown protein interactions) one needs to learn a representation of the graph that is amenable to be used in ML algorithms.

However, graphs are inherently combinatorial structures made of discrete parts like nodes and edges, while many common ML methods, like neural networks, favor continuous structures, in particular vector representations. Vector representations are particularly important in neural networks, as they can be directly used as input layers. To get around the difficulties in using discrete graph representations in ML, graph embedding methods learn a continuous vector space for the graph, assigning each node (and/or edge) in the graph to a specific position in a vector space. A popular approach in this area is that of random-walk-based representation learning, as introduced in DeepWalk.

Left: The well-known Karate graph representing a social network. Right: A continuous space embedding of the nodes in the graph using DeepWalk.
Here we present the results of two recent papers on graph embedding: “Is a Single Embedding Enough? Learning Node Representations that Capture Multiple Social Contexts” presented at WWW’19 and “Watch Your Step: Learning Node Embeddings via Graph Attention” at NeurIPS’18. The first paper introduces a novel technique to learn multiple embeddings per node, enabling a better characterization of networks with overlapping communities. The second addresses the fundamental problem of hyperparameter tuning in graph embeddings, allowing one to easily deploy graph embeddings methods with less effort. We are also happy to announce that we have released the code for both papers in the Google Research github repository for graph embeddings.

Learning Node Representations that Capture Multiple Social Contexts
In virtually all cases, the crucial assumption of standard graph embedding methods is that a single embedding has to be learned for each node. Thus, the embedding method can be said to seek to identify the single role or position that characterizes each node in the geometry of the graph. Recent work observed, however, that nodes in real networks belong to multiple overlapping communities and play multiple roles—think about your social network where you participate in both your family and in your work community. This observation motivates the following research question: is it possible to develop methods where nodes are embedded in multiple vectors, representing their participation in overlapping communities?

In our WWW’19 paper, we developed Splitter, an unsupervised embedding method that allows the nodes in a graph to have multiple embeddings to better encode their participation in multiple communities. Our method is based on recent innovations in overlapping clustering based on ego-network analysis, using the persona graph concept, in particular. This method takes a graph G, and creates a new graph P (called the persona graph), where each node in G is represented by a series of replicas called the persona nodes. Each persona of a node represents an instantiation of the node in a local community to which it belongs. For each node U in the graph, we analyze the ego-network of the node (i.e., the graph connecting the node to its neighbors, in this example A, B, C, D) to discover local communities to which the node belongs. For instance, in the figure below, node U belongs to two communities: Cluster 1 (with the friends A and B, say U’s family members) and Cluster 2 (with C and D, say U’s colleagues).
Ego-net of node U
Then, we use this information to “split” node U into its two personas U1 (the family persona) and U2 (the work persona). This disentangles the two communities, so that they no longer overlap.
The ego-splitting method separating the U nodes in 2 personas.
This technique has been used to improve the state-of-the-art results in graph embedding methods, showing up to 90% reduction in link prediction (i.e., predicting which link will form in the future) error on a variety of graphs. The key reason for this improvement is the ability of the method to disambiguate highly overlapping communities found in social networks and other real-world graphs. We further validate this result with an in-depth analysis of co-authorship graphs where authors belong to overlapping research communities (e.g., machine learning and data mining).
Top Left: A typical graphs with highly overlapping communities. Top Right: A traditional embedding of the graph on the left using node2vec. Bottom Left: A persona graph of the graph above. Bottom Right: The Splitter embedding of the persona graph. Notice how the persona graph clearly disentangles the overlapping communities of the original graph and Splitter outputs well-separated embeddings.
Automatic hyper-parameter tuning via graph attention.
Graph embedding methods have shown outstanding performance on various ML-based applications, such as link prediction and node classification, but they have a number of hyper-parameters that must be manually set. For example, are nearby nodes more important to capture when learning embeddings than nodes that are further away? Even though experts may be able to fine tune these hyper-parameters, one must do so independently for each graph. To obviate such manual work, in our second paper, we proposed a method to learn the optimal hyper-parameters automatically.

Specifically, many graph embedding methods, like DeepWalk, employ random walks to explore the context around a given node (i.e. the direct neighbors, the neighbors of the neighbors, etc). Such random walks can have many hyper-parameters that allow tuning of the local exploration of the graph, thus regulating the attention given by the embeddings to nearby nodes. Different graphs may present different optimal attention patterns and hence different optimal hyperparameters (see the picture below, where we show two different attention distributions). Watch Your Step formulates a model for the performance of the embedding methods based on the above mentioned hyper-parameters. Then we optimize the hyper-parameters to maximize the performance predicted by the model, using standard backpropagation. We found that the values learned by backpropagation agree with the optimal hyper-parameters obtained by grid search.
Our new method for automatic hyper-parameter tuning, Watch Your Step, uses an attention model to learn different graph context distributions. Shown above are two example local neighborhoods about a center node (in yellow) and the context distributions (red gradient) that was learned by the model. The left-side graph shows a more diffused attention model, while the distribution on the right shows one concentrated on direct neighbors.
This work falls under the growing family of AutoML, where we want to alleviate the burden of optimizing the hyperparameters—a common problem in practical machine learning. Many AutoML methods use neural architecture search. This paper instead shows a variant, where we use the mathematical connection between the hyperparameters in the embeddings and graph-theoretic matrix formulations. The “Auto” portion corresponds to learning the graph hyperparameters by backpropagation.

We believe that our contributions will further advance the state of the research in graph embedding in various directions. Our method for learning multiple node embeddings draws a connection between the rich and well-studied field of overlapping community detection, and the more recent one of graph embedding which we believe may result in fruitful future research. An open problem in this area is the use of multiple-embedding methods for classification. Furthermore, our contribution on learning hyperparameters will foster graph embedding adoption by reducing the need for expensive manual tuning. We hope the release of these papers and code will help the research community pursue these directions.

Acknowledgements
We thank Sami Abu-el-Haija who contributed to this work and is now a Ph.D. student at USC.

Source: Google AI Blog


Coarse Discourse: A Dataset for Understanding Online Discussions



Every day, participants of online communities form and share their opinions, experiences, advice and social support, most of which is expressed freely and without much constraint. These online discussions are often a key resource of information for many important topics, such as parenting, fitness, travel and more. However, these discussions also are intermixed with a clutter of disagreements, humor, flame wars and trolling, requiring readers to filter the content before getting the information they are looking for. And while the field of Information Retrieval actively explores ways to allow users to more efficiently find, navigate and consume this content, there is a lack of shared datasets on forum discussions to aid in understanding these discussions a bit better.

To aid researchers in this space, we are releasing the Coarse Discourse dataset, the largest dataset of annotated online discussions to date. The Coarse Discourse contains over half a million human annotations of publicly available online discussions on a random sample of over 9,000 threads from 130 communities from reddit.com.

To create this dataset, we developed the Coarse Discourse taxonomy of forum comments by going through a small set of forum threads, reading every comment, and deciding what role the comments played in the discussion. We then repeated and revised this exercise with crowdsourced human editors to validate the reproducibility of the taxonomy's discourse types, which include: announcement, question, answer, agreement, disagreement, appreciation, negative reaction, elaboration, and humor. From this data, over 100,000 comments were independently annotated by the crowdsourced editors for discourse type and relation. Along with the raw annotations from crowdsourced editors, we also provide the Coarse Discourse annotation task guidelines used by the editors to help with collecting data for other forums and refining the task further.
An example thread annotated with discourse types and relations. Early findings suggest that question answering is a prominent use case in most communities, while some communities are more converationally focused, with back-and-forth interactions.
For machine learning and natural language processing researchers trying to characterize the nature of online discussions, we hope that this dataset is a useful resource. Visit our GitHub repository to download the data. For more details, check out our ICWSM paper, “Characterizing Online Discussion Using Coarse Discourse Sequences.”

Acknowledgments
This work was done by Amy Zhang during her internship at Google. We would also like to thank Bryan Culbertson, Olivia Rhinehart, Eric Altendorf, David Huynh, Nancy Chang, Chris Welty and our crowdsourced editors.

Research from VLDB 2016: Improved Friend Suggestion using Ego-Net Analysis



On September 5 - 9, New Delhi, India hosted the 42nd International Conference on Very Large Data Bases (VLDB), a premier annual forum for academic and industry research on databases, data management, data mining and data analytics. Over the past several years, Google has actively participated in VLDB, both as official sponsor and with numerous contributions to the research and industrial tracks. In this post, we would like to share the research presented in one of the Google papers from VLDB 2016.

In Ego-net Community Mining Applied to Friend Suggestion, co-authored by Googlers Silvio Lattanzi, Vahab Mirrokni, Ismail Oner Sebe, Ahmed Taei, Sunita Verma and myself, we explore how social networks can provide better friend suggestions to users, a challenging practical problem faced by all social network platforms

Friend suggestion – the task of suggesting to a user the contacts she might already know in the network but that she hasn’t added yet – is major driver of user engagement and social connection in all online social networks. Designing a high quality system that can provide relevant and useful friend recommendations is very challenging, and requires state-of-the-art machine learning algorithms based on a multitude of parameters.

An effective family of features for friend suggestion consist of graph features such as the number of common friends between two users. While widely used, the number of common friends has some major drawbacks, including the following which is shown in Figure 1.
Figure 1: Ego-net of Sally.
In this figure we represent the social connections of Sally and her friends – the ego-net of Sally. An ego-net of a node (in this case, Sally) is defined as the graph that contains the node itself, all of the node’s neighbors and the connection among those nodes. Sally has 6 friends in her ego-net: Albert (her husband), Brian (her son), Charlotte (her mother) as well as Uma (her boss), Vincent and Wally (two of her team members). Notice how A, B and C are all connected with each other while they do not know U, V or W. On the other hand U, V and W have all added each other as their friend (except U and W who are good friend but somehow forgot to add each other).

Notice how each of A, B, C have a common friend with each of U, V and W: Sally herself. A friend recommendation system based on common neighbors might suggest to Sally’s son (for instance) to add Sally’s boss as his friend! In reality the situation is even more complicated because users’ online and offline friends span several different social circles or communities (family, work, school, sports, etc).

In our paper we introduce a novel technique for friend suggestions based on independently analyzing the ego-net structure. The main contribution of the paper is to show that it is possible to provide friend suggestions efficiently by constructing all ego-nets of the nodes in the graph and then independently applying community detection algorithms on them in large-scale distributed systems.

Specifically, the algorithm proceeds by constructing the ego-nets of all nodes and applying, independently on each of them, a community detection algorithm. More precisely the algorithm operates on so-called “ego-net-minus-ego” graphs, which is defined as the graph including only the neighbors of a given node, as shown in the figure below.
Figure 2: Clustering of the ego-net of Sally.
Notice how in this example the ego-net-minus-ego of Sally has two very clear communities: her family (A, B, C) and her co-workers (U, V, W) which are easily separated. Intuitively, this is because one might expect that while nodes (e.g. Sally) participate in many communities, there is usually a single (or a limited number of) contexts in which two specific neighbors interact. While Sally is both part of her family and work community, Sally and Uma interact only at work. Through extensive experimental evaluation on large-scale public social networks and formally through a simple mathematical model, our paper confirms this intuition. It seems that while communities are hard to separate in a global graph, they are easier to identify at the local level of ego-nets.

This allows for a novel graph-based method for friend suggestion which intuitively only allows suggestion of pairs of users that are clustered together in the same community from the point of view of their common friends. With this method, U and W will be suggested to add each other (as they are in the same community and they are not yet connected) while B and U will not be suggested as friends as they span two different communities.

From an algorithmic point of view, the paper introduces efficient parallel and distributed techniques for computing and clustering all ego-nets of very large graphs at the same time – a fundamental aspect enabling use of the system on the entire world Google+ graph. We have applied this feature in the “You May Know” system of Google+, resulting in a clear positive impact on the prediction task, improving the acceptance rate by more than 1.5% and decreasing the rejection rate by more than 3.3% (a significative impact at Google scales).

We believe that many future directions of work might stem from our preliminary results. For instance ego-net analysis could be potentially to automatically classify a user contacts in circles and to detect spam. Another interesting direction is the study of ego-network evolution in dynamic graphs.