I just heard on Twitter about another website that let you lend ebooks. Just for the sake of curiosity I went through my posts and gathered a list of all the ebook lending sites I've covered so far.
Counting GoodReads, I'm up to 6 7-8 9. And now I'm down to 7 sites where you can exchange ebooks.
At some point in the future I plan to more in depth study of the topic. As of now I haven't reviewed each of these sites for the simple reason that I cannot lend any ebooks.