Science Fiction & Fantasy addicts love it: Galaxiki the award winning wiki based science fiction galaxy created, maintained and owned by its Community. Membership is free - sign up now and start naming and editing stars, planets and moons, or get your own personal solar system. Go ahead and take the tour.
As a visitor you may explore the galaxy or read science fiction and fantasy stories written by our community. As a site member you may name and edit edit solar systems and write your own science fiction or fantasy stories, post news and stories in our community blog (anything related to science, space, science fiction and fantasy is allowed) and help to build a fantastic online world.
It sounds like a pretty cool idea, but it's still in beta and a quick look around the forums seemed to indicate it isn't the most visited site in the world. Still, I haven't heard of a nerdier social network since Diaspora.