Facebook could just ban political advertising for a year. They don't stand to lose a lot of money, just political influence. It would be a smart move though that would shut the mouths of their rivals in the press.
I think you overestimate how much money is spent on FB political ads. FB had over $16B in revenue in its most recent quarter. Trump and Clinton raised a combined $2B in their 2016 campaigns. If they dumped all of that into FB ads, it would still be only about 3% of FB's annual revenue. Obviously they spent far less, but I don't know what PACs spent.