Posted by Alessandro Epasto, Research Scientist, Google Research NY

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.