Probably x.com or Facebook, Google ads?
No? why not? As far as I can see, those services do not even verify what they show in their ads. There was a thread in this forum where someone posted that x.com approved scam or phishing ads. I have seen many times that they do not verify the services. Scammers promote their services through social media. They care about money only.

So, as long as mixers have money to advertise, it won't be hard to continue advertising. But the problem they may face is, they have to find a targeted audience. Everyone is not Bitcoin users on social media.