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.