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.