Let's see...where's a handy example. Ok...let's use...me.
I joined BookCrossing a month ago. In the past year, I have purchased somewhere in the neighborhood of...oh...20 books. About 12 of those have been in the past month, since joining BookCrossing. These people may be writers, and therefore more adept in English than math...but the math really isn't that hard.
One example (and seemingly a BC favorite) that comes to mind is Chistopher Moore. I never heard of him before finding BookCrossing, but I've already bought 2 of his books, and planning on buying the rest. I may not have bought him a new car, but I certainly bought him lunch! And just by talking about his stuff to friends, I've probably sold an additional 12 or so books of his. But, no, BookCrossing really hurts authors. They hurt their poor hands trying to fold that extra cash into their wallets.