ChameleON Social - Create your own dating site or social network
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
vldPersonals - Full-featured PHP solution for running an online dating site.
phpFox - phpFox is a powerful social network platform for niche communities
Skadate - Skadate is a dating software.
BuddyPress - WordPress social networking in a box.