Re: "what's been revealed about Facebook's operations [..] has harmed investors"
You are right in that Facebook PROFITS have been consistently rising, and I predict that will be a big stumbling block in proving the case.
Stock prices go up and down for various reasons including the general state of the economy or the state of the particular sector the company is involved in. So stock price alone cannot be a factor.
Fundamentally, the bad stuff was done specifically because it led to larger profits. The nastiest part, the dopamine promoting algorithm, is perfectly legal and without liability for fraudulent defamation due to section 230. That is the main point in terms of societal damage, and also the main source of profit.
There is the smaller issue of whether the number of actual unique users was overestimated, and whether that caused advertisers to pay more than they should. That also presumes that advertisers don't have their own indepenent way of measuring ROI on advertising fees. About this point, don't expect more than a symbolic award and a minor change in the language of report number of unique users. I don't expect advertisers to flee - they are there because Facebook really works for them. Proof of that can be seen from the fast FB advertiser response to being cut out of iOS.
In conclusion, the issue of whether "the algorithm" should benefit from protection section 230 is one that that must be decided by lawmakers.