Though the larger sites let users create groups on any topic or interest, finding the right groups and identifying the most dedicated members can be daunting. What happens is a medium reaches a point where the users of it start to think it's interesting but it's too big, there are too many people, it's too difficult to find what I'm interested in?

http://www.cnn.com/2008/TECH/04/16/online.communities.ap/index.html