Your last sentence is interesting. I never considered that AirBnB needs to battle more of this hard-to-distinguish traffic. In another thread on this page, someone else mentioned that -- when logged in with your Google account -- various Google sites/assets never show CAPTCHAS, because it has so much information about your account that they can reliably predict if you are human or robot. I'm surprised they don't offer this as a feature to 3rd party commercial websites. "Add Google login and we can remove 99% of your CAPTCHAS (but you have to be X cents per login)."