Catalyzing Computer-Mediated Collective Action

This was the second time I've seen Marc give this talk and one thing that always strikes me, in regards to Usenet, is this 2% of users who are responsible for such a vastly disproportionate amount of labor in answering questions and replying to posts.

Is there some social science term for this type of behavior or person - or this role they perform in a social network? I'm at a loss on how to refer to them.... - Trav