Ultimately capitalism "works", but only if externalities are incorporated into the price.

Hence vice taxes on liquor, cigarettes, the short-lived Bloomberg tax on soda. See also - carbon pricing.

What would that look like for social media, I don't know. If we're truly brainstorming, what if Facebook were forced to charge you cash money for usage beyond a half hour per day? Or past a certain amount of posting?

I'm well aware that politically this would die even faster than the soda tax... selling a policy is often more difficult and important than policy itself

The main problem the vast majority of policy proposals for this sort of problem face is that the proposals almost invariably slip in the idea of some sort of human being, if not an entire population of humans, that is abstractly above the problem and can be trusted to administer the policy. But if that was the case, we often wouldn't have the problem in the first place.

It's really hard to policy-fix something that literally 99% of the population is doing. Who is going to propose it? Who is going to enforce it? Who is going to pay attention to it?

And to be clear, this is commiseration with you, not argument. I have no solution even in principle.