It's actually a (set of) plugins for wordpress to turn WP into a social network. I've been playing with a test setup for a couple of weeks and it looks pretty solid tbh. How it might stand up to thousands of users is another matter, but for a small to medium scale SN it looks better than any other free option I've come across - I'm discounting elgg because that needs such heavyweight hosting specs to run.
I was going to use it for a free blog site but it adds so much stuff on that the actual core function of WPMU seems to become totally detached from the site. It feels like a social networking site with a multi-user blog platform badly tacked on.
If you're looking for a general social networking script then I honestly don't see the point in it, there's loads better out there but you might have to spend a bit of cash.