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The details of the Cambridge Analytica piece (as I understand them) were that a third party violated the terms of service and grabbed more data than they should have from friends of users that filled out a survey.

Facebook had since restricted access to the API to prevent this terms of service violation a few years ago, but it was after Cambridge Analytica had already engaged in their bad behavior.

The news was largely that a bad actor did a bad thing and FB had already locked down the API to prevent this a couple years ago, though in the news the details around this were mostly incoherent. The market reaction was a massive crash.

It's probably worth Facebook focusing on bad actors more explicitly (which they're doing now with their focus on integrity teams), but the reaction to the CA news was disproportionate - even on HN. The market moves during sensational media stories seem to be an easy place to make money, I think this is where it's similar to the pot smoking CEO overreaction.