From left to right on the Social Networking: Enabling the Two-Way Street panel at MIX07:
Dave Morin, Facebook;
Evan Williams, Twitter;
Aaron Emigh, Six Apart;
Joseph Kleinschmidt, Leverage Software;
Marc Canter, Broadband Mechanics
These guys have all created what Joseph referred to as "Communities of Purpose" (and I am a member of two of those communities myself).