I have started posting all my books to LibraryThing.com. One of the features is that it will tell you whether the book is on a wishlist on one of the top book swapping websites. As a result, I have joined Bookmooch, FrugalReader and TitleTrader. So far, I have mailed 4 books through Mooch and 3 through Frugal. I am expecting 6 books soon.
So far, I like Bookmooch the best because you can accumulate points faster. Bookmooch also allows easier uploading of wishlists and lists of books available for swapping.
None of the sites have very good search search features or sorting of books, so you have to scroll through a lot of junk before you find a book you’re interested in borrowing.
There are usually many copies of popular fiction titles and at least a few of self-help books. Non-fiction books like Freakonomics are in high demand though, so hopefully non-fiction readers will be inspired to start swapping.