WPMU, or WordPress MU. If you know WP (and who doesn't?) you're on your way to creating a formidable and flexible social network, albeit Facebook's patent on the social feed.

I usually go with Ning, because it's extremely easy to set-up and you can customize it if you want. It truly is a powerful social platform. But if Ning doesn't cut it and I need something much more complex, then I go with BuddyPress. It's definitely worth the look.