Goodreads - See what your friends are reading.
This is Broken - A tribute to Seth Godin's book recommendations
Readernaut - We are a small community of readers, sharing our adventures and supporting each other as we persue a life of learning.
Booknshelf - Social book and bookshelf discovery platform.
aNobii - aNobii is a social network for listing books, sharing insights, and meeting people.
Readminder for iOS - Track your reading record and keep your favorite quotes