First there were MySpace and Facebook; LiveJournal is a more science-based community. Now comes the next generation of social networks, both for readers.
GoodReads is a social networking site for readers. Members can post their opinions and reviews of works that they have read, include rating them. For readers it's attraction is that if you're interested in a book, you can obtain a variety of opinions on the same books. That part overlaps with amazon, but it's much more of a 'club' than amazon is.
Galaxiki is a much more curious site. It's "a web-based virtual and purely fictional galaxy project using wiki technology. The word Galaxiki is a portmanteau of the words galaxy and wiki. Galaxiki consists of over 1 million computer generated stars as well as their planets, moons and other orbital objects which are maintained by the Galaxiki Community."
It also posts links to sci-fi & fantasy books and videos, so in that respect, it might be viewed as a GoodReads for Astronomers. I'm not sure of it's long-term viability, but the world needs more variety -- why should Bebo and the like have it all their own way?
So pop on over to both and take a look.
Colin