That’s the thing you don’t understand. There is an argument that there is a right to say whatever you want pursuant to the 1st Amendment on the site if there is state action. And the Supreme Court has ruled before that there is state action due to entwinement between private entities and the government, this creating state action. I don’t know why that’s hard for you to understand. Yes, the Supreme Court hasn’t ruled that the first amendment applies to Facebook, YouTube and google, but based on precedent, there is an argument to be made that it does. Really, go read the Wilmington case and you might have a better understanding of state action exceptions. The go read up on the relationship between big tech and the government.
I’ll link you to a website you’re familiar with.
https://www.motherjones.com/politics/2013/10/facebook-personal-data-online-privacy-social-norm/