SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
BuddyPress - WordPress social networking in a box.
phpFox - phpFox is a powerful social network platform for niche communities
Elgg - Opensource social network engine
mooSocial - mooSocial is the best social network script to create a niche community or social site.
HumHub - HumHub is a free social network software and framework built to give you the tools to make teamwork easy and successful.