Shelfari Gets Lucky, Gets Acquired by Amazon

Amazon has acquired Shelfari, the book-loving social network.
This acquisition will most likely help both sides of the equation.
Amazon needed a book-related social network, and Shelfari was available.
Shelfari needs more users, and Amazon can certainly bring that desire to fruition.

Which Shelfari competitor would have served to be just as, if not more beneficial to Amazon?