Shelfari is basically a book-networking site, rather than a social networking site. Each member has a bookshelf holding all your books, and you label each one as "plan to read" or "reading now" or "I've already read". You can review books and everything as well.
Facebook has an application that is basically the same as it, under the social-living category called "virtual bookshelf". I think it's great for people to share their books and their opinions about recently read books, but the best part of the facebook feature is that it also tells you which of your friends have read the book in question -- I'm sure that Shelfari does this too, but since I just joined and I'd never heard of it before, I don't have a very large network of people to browse.
I'm willing to give shelfari a shot, because I think that any platform for getting out the word on books is a great platform. But I find myself wondering what the function of a site like this is. Its biggest draw is that there are "book groups," which basically function to give people a way to link up with one another based on a variety of topics, such as "Twilight movie and books," "1001 Books to Read before you die," and a variety geared towards high schoolers and younger readers to give them ideas of books to read.
If you're interested in trying out shelfari, the link is www.shelfari.com and it's a pretty easy platform to use. Maybe you already use it, and I just don't know about it.