Goodreads launched in early 2007. And in 2001, per Wikipedia:
In 2011, Goodreads acquired Discovereads, a book recommendation engine that employs "machine learning algorithms to analyze which books people might like, based on books they've liked in the past and books that people with similar tastes have liked." After a user has rated twenty books on its five star scale, the site will begin making recommendations.Users can interact with their friends. The site or app will search through your address book and social media sites to find friends on Goodreads, allowing you to see what your friends are reading, rating, and reviewing. (Is it not an anomaly, but it's more fun for a dork like me.)
Goodreads is also available as an app. It can be used through most devices and Facebook.
Some other similar services are LibraryThing and Shelfari. I have Goodreads on my iPod and enjoy it, so at minimum, I have a tool to collect ISBNs and titles I want to put on hold when I'm on the go. (The Brooklyn Public library did finally release an app this year.) I haven't used LibraryThing or Shelfari, if anyone wants to chime in, I'm all ears (and eyes)!
Goodreads Amazon To Buy Goodreads, And Owns Part Or All Of Its Competitors, Too, Consumerist