Found this great article by Bradley Horowitz of Yahoo!. He talks about the structure of social software services, and dynamics of the community that comes out of it. What struck me is how a small set of users feed and lead the membership. So instead of getting every sign-up to use your service, you should simply focus on your power users. Eventually they will motivate the casual user to become a more active contributor to community.

I guess its the 80/20 rule. 80% of your content comes from 20% of your users. Similarly, you should focus 80% of your efforts to this top 20%.