Recommend a social reading site

I know there are lots of sites that aim to be a social network for readers: GoodReads, WeRead, LivingSocial, and Shelfari seem to be a few of the ones I’ve come across.

Before I invest time in one, marking my collection, etc., can anyone recommend one over the others? There are a few key things I’m interested in, like how easy it is to find and mark books you own (if I find one, will it show me more in the same series/author/genre? Can I import a list from Excel?), whether it integrates with Facebook and how (I’m not a complete hermit … but I might not want to expose my love of Gor, say), and probably other things I can’t think of off the top of my head. (I think I saw that GoodReads had trivia quizzes, which would be a fun feature … but not a deal breaker.)

Feel free to share experiences, preferences, anything at all. Thank you!