But some social media platforms are effective monopolies, as is quite obvious by their market share.
Example?
None of this is speech related, just ordinary trust-busting.
Excellent, so let's trust bust. Let's not allow the trust to function under a new free speech regime. It sounds like maybe we agree here? I get the impression you want to force companies to provide a platform, but here you seem to indicate just having a healthy market is sufficient. If you can't get your idea "out there" in a healthy market, that's not a problem with the market.
But Klan rallies do get parade permits.
Because governments are forced to give them out thanks to the Constitution. That's an entirely different scenario from the (mostly) private venues I'm talking about. It's possible that the Klan could get a city or government-owned venue, but that's only because the government has no choice in the matter (which is a good thing).
The internet is different, and again, would you want a government-run forum?
Why is "the internet different"? You can always inhabit some little dark corner of the internet, just like you can yell at people in Central Park. At least with the internet, you have global reach. And what, exactly, is the difference between a government-run forum and a heavily-regulated private forum?