Goodreads is a social network for those who like to share what they read and find new books to read. I joined Goodreads and was able to easily add a dozen books that I've recently read, and provide my rating and review for each. The site is very easy to use. It's easy to find books to add that you've read and books that you want to read. Goodreads is also easy to find other readers with similar interest and ratings on books that you've considered reading.
With goodreads, you can start your own book club or join an existing one. The groups can even go a little off-topic, such as there is a group for folks to rate and discuss recent movies they watched. You can also find over 4000 authors who participate on Goodreads, as well as some author interviews. If that's not enough, you can also test your book smarts with general book trivia or triva only from books you've read.
Now I'm seeing some value out of social networking... MySpace to keep in-touch with my favorite musicians and Goodreads to socialize about the books I read.