Some number of months back I ran across LibraryThing. It allows you to catalog the books you've read or have on your bookshelf or wish to read all online. The social networking and recommendations aspect comes into play when it comes to connecting with other folks that have the same books in their catalog that you do -- since they may have books in their own catalog that would be of interest to you. Part of the idea is also that you can keep an eye on what your friends are reading, instead of simply forgetting the book they recommended last week. I actually wanted an online catalog for other purposes as well:
- Knowing what books I've already bought and have in a pile somewhere but just haven't gotten around to reading....so I don't buy it again
- Being able to let friends browse my catalog and ask to borrow books
- Tracking what I'm reading so that it can automatically be tracked on my blog for folks that are interested in what I'm reading (I'm usually reading 3-5 books at a time, at the rate of between 2-4 a month or so typically is my best guess).
There seem to be three sites that serve this niche: