Goodreads - See what your friends are reading.
Readernaut - We are a small community of readers, sharing our adventures and supporting each other as we persue a life of learning.
BookWyrm - Social Reading and Reviewing
LibraryThing - A home for your books.
LearnAwesome - Humanity's universal learning map
aNobii - aNobii is a social network for listing books, sharing insights, and meeting people.