I've added a new link to the "SocialNetworking" category of links. It's for LibraryThing, a social networking site about books. It's not the first one (I believe reader2.com (also linked) holds that honor) but I do think it has a lot of potential.
They recently got hit with a tidal wave of traffic, thanks to the WSJ and BoingBoing linking them on the same day. Since they have managed to survive the onslaught (after a few upgrades), their userbase and database have both grown tremendously.
The site allows you to enter your library of books, then rate, review and tag them as you see fit. The possibilities are plentiful from there. You can find people who thought similarly of the book as you did, and then see what else they've enjoyed. You can also see people who's collection is very similar to your own, and then peruse it to see if they've got anything you might be interested in.
Be sure to check out the Zeitgeist link. Lots of good timesinks for readers there.
Also, the site supports importing catalogs of books from other book websites and apps such as Delicious Library.