This is a good point - Youtube/Facebook/Twitter/etc. don't currently have the ability to arrest, try, and imprison you. The bill of rights applies to the US government to prevent it from abusing that power.

Though a de facto oligopoly in the social media space means that your reach could be greatly reduced (though not, as you note, eliminated) if you are deplatformed. I imagine that some politicians, entertainers, and businesses could see large effects.

Being delisted from Google search could also be an issue for businesses due to Google's search dominance.