The idea of BookCrossing appeals to me. You give a book away, directly to someone or left where someone might find it, and by registering it with BookCrossing there's a chance you'll find out who gets it next. Reuse, community, all that stuff. So much so that I joined up pretty quickly (2010-06-07, my account tells me) although I don't seem to have written about it before. I wasn't very active, because I had a hard time finding any BookCrossing-sanctioned sites here in Rome to leave books.
All of which is a long-winded preamble to say that I just this morning received my very first notice that a book I released had been found. I gave it to a friend in 2014, and yesterday someone reported that they had bought it via AbeBooks.
So, what goes around does come around. That little shot of dopamine is enough to make me consider looking again for release sites and setting a few more books free. On the other hand, spelunking around the site makes me wonder if it would be worthwhile. None of the Official Book Crossing Zones in Rome have any books listed, and the two places that do have books listed seem to be unofficial. But maybe this is because I am not a paid member.