So...Publicly-traded Mega-Corporations Are Now Private Companies?

When a celebrity, internet personality or typical social media user gets de-platformed by one of the major tech companies, one of the rhetorical statements is an iteration of "...they're a private company, they can do what they want..." 

Considering the companies that do all of these bannings are either under the umbrella of or are an IPO traded on one of the major stock exchanges. Although I'm just writing this off the cuff  (haven't looked it up yet), I am pretty certain that every one of the major players in social media and payment processing is traded in the stock market or owned by a company that is daily traded. Several of these companies, now that I think about it, were subsidized by the U.S. government, ergo, tax money. 

That tells me that these companies are NOT PRIVATE!!! Does anybody remember when companies got indexed on the exchange they were said to "go public"? Is that term even used anymore? I have some of the worst financial habits and even I know that one. If any person so wanted, provided they had the funds, can purchase a share of any of these companies without the need of the company's approval. That doesn't sound private to me. So, what happens when Twitter, Facebook, Patreon etc. ban and silence one of their stockholders? Some online overlord who doesn't own any company shares gets to tell one of the owners they can't use their own property? When these companies are accepting tax funds which are allocated by the government, that means they are DEFINITELY NOT PRIVATE.