They should be held liable. Just like a TV ad needs to be reviewed, vetted and is ultimately monitored and people can be fined for missrepresentation, why is social media which is not streaming video content any different?
These are deterrences from reality, and if we were forced to take a close look at a large majority of online advertising we would plainly see that it ranges from the outright sketchy, to borderline lying and certainly user-harassing. They worry that if too much scrutiny were applied, they might actually face some consequences for their actions and we might reevaluate their position as arbiters of “what’s viable” on the internet.
Facebook just isn't one of those I guess.
Billions of not-ads get served on Facebook yet they were able to clamp down on stuff like ISIS uploading decapitations.
imagine if the FDA or some other sort of stringent agency had that kind of remark. "Well, we make 3 billion pills a day, of COURSE 15% of those are going to be deadly poison." -- wouldn't fly for a second.
the excuse of "Well it's going to be real tough to reduce the amount of poison pills while still making trillions in profit" would be equally useless; society wouldn't (and shouldn't) care about that.
A simple solution is requiring the companies providing the ads to be placed to indemnify the ad placers. That means the ad placers push the monetary liability onto their customers to only provide legitimate ads. As they know exactly who their customers are and can vet them it is trivial for them to go after them if they are sketchy.
Easy. Next problem.
"Oh gosh, I opened the door and a bunch of rats came in... I haven't closed it yet. What can I possibly do to take care of this rat problem?"