I'm sympathetic to the argument that Facebook/Twitter/etc. are too large and have too much power for lobbying/influencing public discourse, although I think if anything making them a public utility would make that situation far worse as opposed to just breaking them up or something else to make the market more competitive

But also just because they are big platforms why does that give people a right to be on them? Is my speech less free because I have a smaller audience?

So this is all about, as another commenter said, complaining that you can't get the largest audience for your mole people rant. Facebook and Twitter are both very large social hubs, but they still get to pick what they publish. They are giving you the ability to publish anything you want until enough people (or the right people) complain about it.

It is democratized moderation. If your following is small enough to skirt the mods, then you can post what ever you want. If your following is huge and you post a bunch of lies about sewer mutants, or that the covid vaccine gives you rabies, or that Hillary Clinton is actually a space alien in cahoots with Planned Parenthood to subsist off the flesh of aborted 6 year olds, then YES, they will remove your posts, and potentially ban you for a period of time.

This literally happens to my aunt every few weeks. She gets a weeks long ban for basically reposting only Russian spam, gets her account back and does it again. It has never even been permanent.

Okay, but how is the situation worse than it was before FB and twitter existed?

The amount of eyeballs available today for even small sites is far greater than it used to be pre-facebook.