Elgg: Open-source social network platform

[KenR]

Elgg is an open source social networking platform based around choice, flexibility and openness: a system that firmly places individuals at the centre of their activities.
Your users have the freedom to incorporate all their favorite tools within one environment and showcase their content with as many or as few people as they choose, all within a social networking site that you control.

http://elgg.org/

from http://www.newsforge.com/