Obviously only a small percentage of books released ever write home. But to hear back from those few is very exciting. You can help your catch rate by labeling well, making sure the BCID and URL are legible and what to do with a catch is understandable. And of course the more books you release, the more likely you are to hear back from some.
I love BookCrossing more every year, but it has always been very precarious financially. Now that I have a lot of books out in the world I worry that if BookCrossing doesn't have the money to continue, my books will become orphans, unable to write home.
So I am very happy to support BookCrossing through label and sticker purchases from the Supply Store and directing any purchases I make through their partner and affiliate links, not all of which are Amazon links. http://www.bookcrossing.com/buy
In fact, the preferred link is with Better World Books.
They also make a small amount from ads. Although BookCrossing remains free, you can eliminate seeing ads by purchasing wings for as little as $5 a month (I think) or by making purchases.
Very few of the members make donations or make purchases however and I personally do not begrudge BookCrossing taking ads to stay alive. I'd be devastated if the site failed. I guess if enough members supported the site it would not have to take ad revenue, but at this point it does have to, to continue to exist.