Customer service is something that doesn't scale well, so requiring good customer service is probably an advantage to small players.

This is also the intersection of customer service not really working at large scale, and residual payments (royalties) not working at absolute scale.

I think this is a major (the only?) reason these internet companies are even successful. Because they just ignore all these costs that don’t scale well that ordinary companies should have.

I think that is a touch strong. Though, I agree that largely nobody really factors in costs of scale, as absurd scale just isn't something that makes sense. This is true in physics as much as it is in economics. Things that work in one scale do not in another. Things that work in changing scale don't work in fixed scale.

What if the law says "social media companies with more than [quantity] in revenue" ?

Or, something to that effect?

This is what I would immediately suspect to start. Probably a good start, even. Does run into complications of jurisdiction and hazards of growth. None reasons not to do something.