A brief definition of Web 2.0 from Tim O'Reilly:
"applications that harness network effects to get better the more people use them"
Web 2.0 Compact Definition: Trying Again
So, for example, Yedda attracts people who contribute questions and answers. and invites them to answer questions based on their interests ... which leads to new answers, new questions, and new invitations. Users of Yedda are connected through their interests; and the more people -- with more interests -- are part of the network, the better Yedda can be. (I know I'm simplifying how Yedda works ... but I believe that's the basic concept.)