You ask the user to say what type of ad it is, under penalty of violation of contract, and contractually obligate them to carry the legal risk if they lie.

There's no requirement that it be done via some fancy ML algorithm. I would expect that unless FB actively attempts to piss off the government, that would pass legal muster. Goodness knows I've signed enough agreements where I've agreed to very similar clauses just in normal day-to-day life.

The same way that newspapers, television stations, radio stations, magazines, and advertising agencies have for the last 200 or so years.

And they didn't have super-duper AI, the smartest people on the continent, and warehouses full of servers to help.

By not permitting Airbnb URLs as generic ads.

Edit to ad: and placing a warning if it looks like it might be a housing ad: “this looks like a housing related ad and has been flagged for moderation”

By doing it.

Facebook loves to try to play both sides, and claim to paying customers that it absolutely has the ability to do a thing, while turning round and saying to regulators that it is only a poor helpless social-media network utterly lacking in the ability to do such things.

If even a fraction of a fraction of a fraction of the capabilities Facebook claims in its materials for advertisers are genuine, it can figure out which ads are housing ads.