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).