Goodreads

I started a Goodreads account first to simply keep track of what I'd read. It comes in handy if you're one of the many librarians who suffers "brain freeze" in reader's advisory situations...or in that embarrasing situation where someone meets you at a party, finds out you're a children's librarian, and says "Oh! Perhaps you can help me! My nephew is turning 12 next week..." 

I quickly found out how fun it was to connect with a handful of friends and colleagues and to see what they were reading...and what they'd thought of books I'd read.   This is social-networking stripped down to its most basic for bibliophiles and offers a variety of ways to participate.  You can use "shelves" as labels to group your titles in various ways.  You can review books (and "follow" each others reviews) and recommend books to friends...join groups that work as online bookclubs, or create or vote on lists.