Freedom of association is rooted in freedom of speech. Otherwise all the Civil Rights Acts would be unconstitutional as they fundamentally impact association, but not in a way that dilutes speech which is what freedom of association covers, for better or for worse. So, social media corps lacking freedom of speech rights in their users content, they also lack freedom of association rights with regards to that content and those users. To push the metaphor too far, their users' speech and their own speech are like oil and water: they are too different to dilute each other.

Now, would an alternate universe in which freedom of association was actually a robust right on its own be a better one overall? Would I happily trade that for this social media thing? Quite possibly, quite possibly.

That’s not freedom of speech. Freedom of speech only protects what you say only from government censorship.

That's not freedom of speech. Freedom of speech is a general principle that covers more than government censorship. What you're thinking of is the first amendment, which covers government censorship (including by its agents). 1A neither invented nor set the absolute boundary of the principle of freedom of speech.

In that case freedom of speech is generally defined according to Mills in On Liberty which still doesn’t acquiesce to the application suggested of social media. They as a large business of many users should not be provided a greater speech mandate than a dissenting individual should speech be actually free.