Riffle is a new social book site to help people discover new books to read. I tried to take a look, but it wanted me to sign in with either Facebook or Twitter, neither of which I wanted to do. However, a quote from Publishers Weekly suggesting it could be a Pinterest for discovering new books was a bit of a turnoff. Another book discovery site is Bookish. The publishing industry is up-in-arms about Amazon buying GoodReads, so expect more of these types of sites to pop up as publishers look for more alternatives, even creating their own.
If that sort of thing is your thing, you might want to give it a look, but I'll stick with LibraryThing. I'm more interested in keeping track of my books than finding more. I have many other ways to find good books to read.