TeensReadToo: A place to learn about great, new reads quickly and easily.
St. Martin's Reading Group Gold: Though not updated regularly, this is a good site for reading groups and a place where you can get free books. (Everyone wants free books, right?)
Readit Forward: The easiest place to get free books! Each month, they have three options of Advanced Reading Copies that you can choose to be sent to you (first come, first serve). They don't expect any review or comments (though it would be the polite thing to do ;).
RandomBuzz: Teenagers can earn points by participating in online activities and after acquiring so many points, they can spend them in the RandomStore (includes books, a gift card, etc.).
LibraryThing: You can post reviews, add books to your wishlist, ask to review books, and more! This site is mostly used to keep track of your books.
BookDivas: Doesn't matter if your a boy or girl, this is a place where you can share your enthusiasm about all the young adult books out there! There are contests posted regularly and interviews with some of the most well-known middle grade/young adult authors.
AuthorsNow!: Contests, posts about books coming out months from now, and a place where you can talk with authors one-on-one. How could you pass that up?