Shelfari is a site designed for social networking aimed at booklovers - all kinds of books for all kinds of people although probably mainly adults. The site has groups, forums, discussions or you can just read about what other people are interested in. The purpose is books and the audience, people who like books. Amazon acquired this site which means a person using Shelfari can feel confident that the site is authentic. It is free ( although Amazon would hope to sell some books, no doubt) easy to use and has a wide range of titles. They also feature reviewers from well known papers such as the New York Times.
The other site I looked at was Encompass the Global Bookclub. While initially the site does not appear very dynamic it has some lovely navigation features designed to appeal to their intended audience; i.e. readers of all ages from all countries. The links are divided into 3 groups - children, 12-18 and then adults. Navigating through the links is easy and fun with interactive graphics that help you define the topic of the book you are looking for. The site is run by the British Council Arts Group and seems to have been developed to instill a love of reading and a way of encouraging people to explore different fields in the arts.