I think we've lost a lot of participants because we're not as unique as we used to be. There are so many other sites where books are traded and shared now. And, while none of them want to make the world into a library, I'm not sure we do, anymore, either. It seems like that goal gets lost in everything else, to me.
I still advertise for BookCrossing, still release books, still do major giveaways at Halloween. Once in a while I get a catch, though usually second-hand. I know the books are taken and read and, as always, that's more important to me than getting tallies on my bookshelf. I can't say it's politics because the slowdown began quite a while ago, but maybe the anti-intelligence atmosphere affects reading books?
I was entranced by BookCrossing when I read an article, just a short piece, in Book Magazine, back in 2002. Have we had much media attention lately? I'm out of the loop enough, you could have an article published today and I wouldn't know. But, did you? And, lack of advertisement - well - that could cause a slowdown. Also, let your people know how they can help - you have a huge force at your disposal.
Glad you're back.