Is the content hosted on social media platforms considered to be the speech of the companies that own said platforms? If yes, wouldn't that undermine the case for their immunity from liability from that content?
> It also means that I do not have to carry, parrot, amplify or provide a space for their speech.
You don't, if you're a private individual, but if you're a huge company that provides a space that's become a de-facto public square then you do, because if you don't, you're affecting freedom of speech in a meaningful way by providing everyone apart from a certain few individuals with a giant megaphone, pushing up the background noise level.
To clarify: Twitter and FB are de-facto public squares, this is fact.
Free speech includes the freedom to be heard, and giving everyone a vuvuzela apart from a few prevents those people from having the freedom to be heard.
Of course - if you're such a company - you also have the freedom to not do business at all. Which would probably be better.
Freedom comes with a cost, but using a bludgeon like the US Supreme Court to outlaw "bad speech" today may have severe repercussions tomorrow. We should look for other solutions. Social solutions for social problems.