One of the social networking sites dedicated to readers and book-lovers is called Shelfari. I recently joined it, and added myself as a Shelfari author. You can see my profile here.
I am already on Goodreads and LibraryThing, but one reason I bothered to join Shelfari is that it is owned by Amazon; when authors add info to their books (i.e., books they wrote)— like characters' names, story setting, and such— on Shelfari, Amazon creates a link to the Shelfari info on the book description page on Amazon. This gives authors a chance to get more info about their books to potential readers. Plus, Shelfari is one more group of people who love to read, and I am all about readers.
Check out the Book Extras section on the Amazon page for The Sixth Discipline and you'll see the link to Shelfari.