The book swap

So here I go... And like many projects, this book one requires input from knowledgeable people - like you!

I have limited time - I also have limited space. So I don't have enough room to hang on to every book I've ever purchased or received. My equation for many years has not been exactly quantified, but can be said to go something like this: If I've read the book more than twice, and plan on reading it again, it deserves to stay. Otherwise, it's time to go.

Because of my role as book reviewer, because of some well-intentioned gifts, because of a tantalizing library book sale, I've received a few books that do not pass this test. So I have a number of books that need to go. The question is: how?

In the past I've donated to the library, or traded in at a used book store. Right now, I'm interested in looking into book swapping. Does anyone have an opinion on the book swap websites out there, or even just the concept of book swapping? All input appreciated.