Goodreads - See what your friends are reading.
GoodBooks.io - Largest curated collection of 8,500+ book recommendations.
BookSloth - Book Sloth lets you add books, post and see reviews as well as interact with other readers.
BookAuthority - BookAuthority collects the most recommended books on business, technology and science - as featured on CNN, Inc and Forbes
LibraryThing - A home for your books.
BookWyrm - Social Reading and Reviewing