It would be outrageous for FB to run the bots themselves. There's two other issues that are more realistic and unfortunately not so great for FB: they're aware

It would be outrageous for FB to run the bots themselves. There's two other issues that are more realistic and unfortunately not so great for FB: they're aware of the bots and ignoring them in favor of the ad revenue or they're just becoming aware of the bots and will implement some fix which could result in a substantial decrease in ad revenue. Good to keep in mind that OP's 80% is just one data point and may not be accurate.