Punishing companies directly that do business with Facebook/Meta would probably be the strongest deterrent. If we can sanction countries, why not companies?

That's how sanctions are usually implemented. See: Russia and Iran, trickling down to their state businesses and armaments industries, tricking down to their suppliers. The only difference here would be there isn't a nation-state as the starting point.

Sorry, I meant the other way around. Sanction Meta and subsidiaries, and then punish companies doing business with them.

Yup. That's what I meant too. The Iranian Revolutionary Guard Corp doesn't care much about US sanctions, but other companies selling them {insert thing} do, if it has financial repercussions.

I bet Facebook will still happily provide service in a region, even if the only customers for their ads there are groups like companies with no footprint in the country, foreign regimes that would like to influence elections, and locals who’ve figured out ways to evade local laws.