WordPress supports BuddyPress, goes social.
Wordpress MU, the multi-user version of the Wordpress blogging engine powers gazillions of blogs on Wordpress.com and is to get social networking features following Automattic's hiring of BuddyPress creator Andy Peatling.
The BuddyPress site is currently moving servers but you can still get the open source BuddyPress code from Google Code.
I just had a quick look at the code; the core features (profiles, friends, and messaging) have been implemented and it looks promising.
I'm still debating which (if any) existing platform to base the social networking aspects of Singularity on. The top contender so far is Drupal but, the more I look into existing solutions, the more I feel that a clean, service-oriented custom solution is the best one for the Flex-based Singularity conference application.