If any of you are interested in trying out Riffle (a book-themed Facebook add-in), you can use this direct access link to sign up. This link is good until Saturday morning only. Riffle lets you create booklists and share them on Facebook. You can also follow people, and see what they are reading, recommend books, mark books as read, etc. I've been dabbling with Riffle for a few months, and some new features should be coming on line soon.
One Riffle thing that I have found to be a potentially addictive time sink are "Questions". You answer questions like "What books have changed the course of your life?", and you can see what books your friends and others have selected in response to the question.
There are some changes that I would still like to see that would make Riffle more useful to me as a reviewer. But it's still evolving, and I've been giving it a try. If you would like to try it yourself, you can sign up here.