There are lots of member pics on instagram. Look up #bookcrossing. AS of this instant there are 10.664 posts using that hashtag, and lots of members posting. It seems to me that it's the members that are doing the actual BookCrossing. That's where the action is.
I'm not sure a lot is members. My impressions is that a lot of people call it bookcrossing, but is not real bookcrossers. They just release books. You can look at the photos and hardly any have BC stickers or labels. They spell bookcrossing wrong. Lots of photos come from east-Europe and Asia. So I think bookcrossing should be there to promote the real thing. I once heard someone say thet they know several people who did bookcrossing, but had never heard of bookcrossing.com.