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.

https://investor.fb.com/investor-news/press-release-details/...

https://www.washingtonpost.com/graphics/politics/2016-electi...