Goodreads - See what your friends are reading.
LibraryThing - A home for your books.
Readernaut - We are a small community of readers, sharing our adventures and supporting each other as we persue a life of learning.
BookAuthority - BookAuthority collects the most recommended books on business, technology and science - as featured on CNN, Inc and Forbes
aNobii - aNobii is a social network for listing books, sharing insights, and meeting people.
GoodBooks.io - Largest curated collection of 8,500+ book recommendations.