I wouldn't attribute any of the downfall of Couchsurfing to AirBnB, rather the new owners. There are a ton of reasons which were laid out by multiple disgruntled ex-couchsurfers in a myriad of blog-posts. I went looking for some of them but the best I could find was on Quora[0]. For me it was apparent that they new owner only cared about the number of users and proceeded to advertise instead of relying primarily on word-of-mouth, ignoring critical feedback from core members/ambassadors, as well as destroy the usability of the site (e.g. search and the groups got progressively worse), among reasons. The site was rather quickly overwhelmed with people who just wanted a free hotel while they go out and party, while the core community abandoned the site, leaving a vacuum to be filled by people who didn't care about the original "spirit". I went from hosting people every week, making lots of spontaneous trips and organizing events to not being involved at all. This was also true of tens of CS friends, many of whom pleaded with the new management to stop breaking things and to listen to their feedback. I knew at least 15 ambassadors personally who deleted their profiles around the same time all these changes were happening, particularly because their feedback was being completely ignored. I kept my profile up in hopes that things would eventually return... and also out of curiosity about how bad it could really get. Only last month have I received the first "worthy" request, where somebody actually read my profile and seemed to have some of the original couchsurfing spirit. Perhaps things will change again?
[0]: https://www.quora.com/Is-CouchSurfing-dead