a) historically it's done a lot of shady stuff and gotten a pass on that,
b) it really screwed up around election time, with major / unprecedented geopolitical and economic consequence,
c) it doesn't seem to have fully understood let alone fixed the problems,
d) it's business model is interfering with fixing these problems, and
e) it's managed this situation from a PR perspective very poorly.
All of those are facts on the ground that would suggest FB is operationally not as solid as people previously assumed, and that it has many (possibly inherent) vulnerabilities that are not easily fixed that add risk to the business - risk of people abandoning it, risk of regulation, risk of being manipulated for political purposes.
Certainly FB was used by those looking to manipulate public perceptions during the election (and since), and that makes it in a certain light a 'victim'. But they were also the creators and popularizers of this new way of being manipulated, and thus an excuse of being ignorant of the implications is a bit weak, especially because of how they've handled things since.
Just practically, it's extremely unlikely that independent news & research orgs are all conspiring to manipulate FB's stock price. At some point people stop giving companies or individuals benefit of the doubt, which is what's now happened to FB.
This pattern of bad press you're seeing with FB is going to happen a lot in the future with our 24/7 news cycle and rumor mill. Something will come out about a company or individual about them doing something bad that will get a lot of visibility. In that process a lot of their past transgressions will come out, or be re-evaluated in a new light, and it will happen through a slow drip of new discoveries that will keep them in the press for a while. This will break into mainstream discourse, and it may permanently damage their reputation. This is a pattern inherent in the mix of people/companies royally screwing up, the existence of deep digital info trails, social media giving a voice to every person to share juicy info, and the instantaneous social media news cycle.