Goodreads - See what your friends are reading.
aNobii - aNobii is a social network for listing books, sharing insights, and meeting people.
LibraryThing - A home for your books.
BookSloth - Book Sloth lets you add books, post and see reviews as well as interact with other readers.
Rocket Reads - A simpler alternative to Goodreads
Read This Twice - Verified book recommendations from people we look up to