SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
phpFox - phpFox is a powerful social network platform for niche communities
mooSocial - mooSocial is the best social network script to create a niche community or social site.
BuddyPress - WordPress social networking in a box.
Diaspora - The online social world where you are in control.
Hiveflare - Hiveflare is a platform that lets people easily create a social network in minutes.