Here's their blurb from the home page:
Have you ever wanted a better way to:
* see what your friends are reading?
* keep track of what you've read and what you'd like to read?
* get great book recommendations from people you know?
It's not just a place to list the books you read, but a way of comnparing books with friends, finding new titles, and just general social networking through books. I haven't used it a lot yet, but I like the idea!
I hype this site pretty often - it's an e-mail book club (actually, a dozen of them, and you can sign up for as many as you want) that makes weekly selections and e-mails you daily excerpts. It's a convenient way to sample books, and I've found a lot I wanted to read further after getting that taste.