I've been using LibraryThing and Twitter for a while now. I added both as widgets to my blog a while ago, too. You can see them to the right!
I like LibraryThing because it is allowing me to track my reading. What I don't like is that it doesn't allow for showing how much of the book you've read. For instance, I'd like to be able to post a book and then show how far I've gotten in the book when I go to update. I can do that with my knitting projects on Ravelry.com, and I like it! My reading isn't usually just one book at a time, ya know? I've heard some folks talk well of goodreads.com, so I might take a look at that soon. I just don't want to get engulfed in yet another social online site. They become so time-consuming!