Interesting idea - social networking site to discuss books - http://www.bookdiscussions.com/

(new, and a work in progress)

There are add-ons to Facebook, etc. that let you show people what you are reading or have read, but this looks to be a place to talk about books.

I would like to see an idea like this integrated into library catalogs some day (or at least an Amazon-style "review" feature).