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.
Trump and Clinton spent $81M on US election Facebook ads in 2016
https://techcrunch.com/2017/11/01/russian-facebook-ad-spend/
and
https://www.vox.com/2018/10/23/18015228/facebook-top-politic...