Group Formation in Large Social Networks

L. Backstrom, D. Huttenlocher, J. Kleinberg and X. Lan. “Group Formation in Large Social Networks: Membership, Growth, and Evolution”, Proceedings of KDD 2006.

Datasets on membership in LiveJournal groups and explicit “friend” relationships; and on publishing in conferences and explicit citations between authors.

Question 1: How does the probability of joining a group depend on the friends who are already in it?
A: ‘The data suggest a “law of diminishing returns” at work, where having additional friends in a group has successively smaller effect but nonetheless continues to increase the chance of joining…’ But if a greater percentage of the friends are linked to each other, the probability of joining is even higher. They suggest that a “strength of weak ties” argument would suggest the opposite of this finding (you’re more likely to find out new info from weak ties who don’t know each other). But I think decisions about joining require much more than just finding out about the community. (See next blog entry on what makes people commit to/stay in a community.)

Question 2: Which communities will grow over time?
A: Here the characteristics provide a little less predictive power. One obvious one, given the result above, is if there are a lot of people who have a lot of friends in the group, then the group will have larger growth in the next time period. Somewhat more puzzling is that the more three-person cliques in the group, the less the group grows. This could reflect that stagnant groups eventually develop more links among members and hence more cliques.

Question 3: “given a set of overlapping communities, do topics tend to follow people, or do people tend to follow topics?”
A: More frequently, people active in a conference where a topic is hot start going to other conferences where the topic is already hot, rather than the transplantation of people causing the topic to become hot.


About Paul Resnick

Professor, University of Michigan School of Information Personal home page
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