1) the scammers came.
2) governments started holding Google accountable for running ads for online gambling and prescription medication. Google (and Microsoft and Yahoo) had to pay large fines for running ads for products and companies that governments saw as illegal.
As the years passed, Google has become ever more restrictive in what ads it will run. To the point where trigger-happy Google Adwords staff whose job is to approved ads reject many ads that are actually fine.
I think Facebook will go down the same path.
My main product is software used by poker players to track and analyse their winnings and losses. I haven't been able to advertise on Google for years, because the ad reviewers see the word 'poker' and demand proof that I am certified to running a gambling company in the locales I'm advertising in. I've given up trying to argue my case to Google that just because my product has the word 'poker' in the title doesn't mean I'm running a casino!
Facebook, however, still approves my adverts within a few hours. I'm expecting this to stop as Facebook tightens up after a few large government fines.