Online communities I've played around with:

- [[http://www.friendster.com/][Friendster]] 
- [[http://www.orkut.com/][orkut]] 
- [[http://copainsdavant.linternaute.com/][Copains d'Avant]]

Now, if all of those could be based on [[http://www.foaf-project.org/][the friend of a friend (foaf) project]].