Librarything seems fun and quite novel (pardon the pun, that was actually an accident!). I always tend to lose track of what I've read and am always looking for recommendations on what to read next (I also like NoveList for this!).
Feel free to check out my librarything account ...I haven't added many books yet. I'm not sure I would get on board with the "social" aspect of this site, but I like being able to click on a book that I like, see who else likes it and what other books they like. I suppose this could be considered social, at a stretch?
The LibraryThing for libraries feature looks rather interesting too!