An article from boxesandarrows:

  1. What’s your social object? Make sure there is a “there” there. Give users a reason to rally. Why would someone come to your site?
  2. Give people a way to identify themselves and to be identified.
  3. Give people something to do.
  4. Enable a bridge to real life (groups, mobile, meetings, face-to-face).
  5. ently Moderate. Let the community elevate people and content they value.