#1 - Eliminated all paid search other than some limited branded search terms and shifted all the money to affiliates who were way better at making profits on the keywords we were competing on
#2 - Eliminated all display advertising after running numerous experiments showing it provided almost not incremental conversions, even though the platforms happily took credit for them.
Those two things drove our blended CAC down substantially and by building better affiliate relationships, sales actually increased.
The lesson here is that you need to try a lot of things out and you should be continuously questioning what you're doing and running specific experiments to gut check effectiveness of any ad platform that is slapping cookies on wide groups of users and claiming conversions.
My suspicion is that this is near impossible at any large organization, even one as new as Airbnb. I can just imagine someone walking into a team of 20+ performance marketers and suggesting they need to experiment to determine if any of it is remotely effective. COVID forced them into this but it's something that they should have already been doing.
I also suspect that the top line focus/obsession of most VC-backed companies make this type of exercise seem almost counterintuitive. Don't mess with or question the momentum.