To-day I've started playing around with Librarything.com. It is a place to list books that are in your library or that you have read. You can write book reviews and expand some of the common knowledge of books you are familiar with. Additionally, there are numerous groups and ways to connect with other book lovers.
It's easy enough to sign up. You don't even need to provide an email address (but you might want to include one in case you ever forget your password). No personal information has to be given. The free version of the site allows the listing of 200 books. the paid versions (either annual or lifetime, lifetime being very reasonably priced) can list an unlimited number of books.
Books can be assigned to different categories: Your Library, Wishlist, Books You've Read, To-Be-Read Pile, etc. You can categorize your books in whatever way is most helpful to you.
There are over a million users of the site and over 40 million books in the database. After you start to enter books you will see listings of other members with similar libraries to yours. There is also a lot of cool stuff over in the Statistics/Memes section of your profile.
There are also local resources available. Local book groups, user submitted listings and reviews of local book stores, and other things to make the site more useful for your real world interactions.
Then, of course, there are all the widgets and ways to connect to other sites such as Facebook, Bookmooch, Paperback Swap, and numerous others. Personally, I'm not liking my Library Thing account with any of my other social media sites; sometimes you don't want your online identities to be too interconnected.
If you are a book lover and want to connect with others Library Thing is a great resource.
The only thing I have to decide now is if I'm going to start listing all of the role-playing game books I've read over the years.