Goodreads - See what your friends are reading.
What Should I Read Next? - Book recommendations
LibraryThing - A home for your books.
Bookicious - Find the best new book to read with books collections for makers, founders and entrepreneurs.
BooksReddit - BooksReddit is a social book recommendation aggregator.
BookSloth - Book Sloth lets you add books, post and see reviews as well as interact with other readers.