Making publication easy on social media has certainly had an impact on public speech, but private platforms do not offer free speech by design.

Naomi Klein went into this in No Logo with shopping malls replacing public spaces where you also don’t have a right to free speech and can be evicted arbitrarily at the owners discretion.

You’ll find virtually all of social media platforms have moderation, usage policies and user banning practices that go well beyond allowing the fully legally protected free speech you are afforded in a public space (in many countries).

If all spaces that attract the majority of people are private and have homogenous terms of use, then free speech ends in all ways except on this technicality.

Edit: removed unnecessarily inflammatory phrasing.

It (practically) doesn't matter what the moderation policies are, a legal ban on social networks will still be a restriction on free speech

What access to tools or avenues for speech should fall under the first amendment then?

This is a telling argument. Newspapers and television broadcasts, while geared towards broad public consumption, were never wholly democratized platforms and that didn't run afoul of the first amendment. It stands to reason that management of content on social media platforms or outright banning the same wouldn't either.

The restriction that this freedom is supposed to save you from is that of prosecution. Nobody is promising everyone a megaphone.

And this whole conversation is about laws mandating something and the resulting prosecution with comparisons to safety standards in saws, not your made up megaphone