GoodReads, GoodReads, GoodReads. I love GoodReads and far prefer it to LibraryThing. The main reason: there's no limit on the number of books I can add to my account. Also, I get email updates on what my librarian friends around the country are reading. Shallowly, it has a prettier interface.
Which isn't to say that LibraryThing is without it's benefits. I will turn to LibraryThing occasionally for a hard to find book when the details I have are "it's a blue book about a princess". Thanks to the tag search I've been able to find a few titles this way.
It would be interesting to set up online book groups on one of the sites. (Again, I prefer GoodReads.) I'd recommend it for personal use to teens, but can imagine lowering that age when using the site in conjunction with a book club. If we didn't already have an electronic summer reading program I imagine we could create a group in one of the services that would function in a similar way, at least on the patron's side.