They all seem to confuse the concepts of owning a book, having read it, and liking it. Not totally, but enough to make the sites less usable than I’d like.
Let’s face it: We all (at least those of us that work for publishers) have plenty of books we’ve never got around to reading. We all have read books we didn’t own (whether borrowing them from a library or a friend). We all have read books that we definitely don’t want to read again, and that definitely shouldn’t be used for recommendations.
As I see it, the three concepts are different, and should be treated differently. I might want to catalogue the books I own so that people don’t buy me them for Christmas, or so that people can offer to buy one of them. I might want to list what I’ve read recently so that people can chat with me about them. And I might want to list the books I love so that people can recommend me other books or decide whether we share tastes.
Actually, these three concepts could be extended to many products other than books, such as music, food, clothes, etc. Perhaps somebody should make an “I like this” website?