Back in July of 2007, Andy Peatling posted on his blog about creating a social network called ChickSpeak on top of WordPressMU, the multi-user version of the popular open source blog platform . It was a compelling possibility – compelling enough that Automattic (the company behind WordPress) hired him to work on it full time.
BuddyPress 1.0, the result of that effort, was released on April 30th of this year, and is an option that should be seriously considered anywhere a new content-centered community is being built.