http://www.goodreads.com/
http://www.librarything.com/
Goodreads seems like a good place to try the concept and Librarything lets you start with 200 books before money is involved.
Goodreads asks for access to your addressbook on Googlemail for example. This is similar to Facebook which makes this request in order to find friends who are members.
You can decide if you trust them not to use your addressbook information to sell to spammers.
You can check out their privacy policy here.
A books social networking site could tie in well with the idea of a book exchange when visiting with people.
You can do some web research beforehand to see what books would be best to exchange.
Also could be a good excuse for a conversation ;)
I also highly recommend a free book cataloging program called Books to those of you who are fortunate enough to have a Mac.
I haven't tested import/export between Books and Goodreads yet but it looks like this can be done with tab delimited text.
For those with Windows or Ubuntu (Linux) here are some options for you.
(I haven't tested these)
Windows: Bookie
Ubuntu: Alexandria
Technorati Tags: Books, Entertainment
