I believe a lot of Bookcrossing traffic died due to changes at the US Postal Service. Rate increases in general and the loss of book rate for international mail made the trading and RABCK aspects of Bookcrossing far more expensive. It was very attractive to me that by spending around $3-5 I could arrange a trade with another Bookcrosser and I would get a new-to-me book and they would get a new-to-them book, or surprise someone with something from their wishlist. But with the changes at USPS, mailing a book abroad is now around $12-15. I can buy a new book for that (or several used books), so I rarely trade or RABCK internationally any more. My trading pool is now limited to US only, and that pool appears to grow smaller. I suspect attrition at Bookcrossing is because many other members also found making trades here a good deal until the mailing rates changed and it was no longer cost effective. As you cannot change the USPS’s offerings, I do not know what you can do about the loss of the use of Bookcrossing for the purpose of trading books.
No one needs to remind me that the original intent of Bookcrossing was to wild release the books, but honestly and practically, the trades kept me invested. Considering how infrequently I hear back from wild releases, I could release my books without any labeling (i.e., not Bookcross them) and hardly notice a difference. Additionally, the trades/RABCKs generate journal entries and you get the emotional thrill that your book went to someone who wanted it. People in Finland and Germany want the books that I have! But instead, I will release them here and get a JE saying, “I found a book,” once or twice a year if I am lucky. I like the idea of Bookcrossing, but do not need to spend money to give my books away. I do sometimes question sticking around because the tangible and emotional benefits have greatly decreased over time.
That said, I'm looking forward to your efforts to reenergize the site and am happy to see your commitment to improving Bookcrossing. Hope others are more constructive than I am.