THINK thenlive - Location-based social media
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Minds - The open-source, encrypted social network that expands your reach for using it.
phpFox - phpFox is a powerful social network platform for niche communities
Hiveflare - Hiveflare is a platform that lets people easily create a social network in minutes.
BuddyPress - WordPress social networking in a box.