We realised things like lead in toothpaste (or whatever the fuck other whack "medicine" they came up with decades ago) were harmful and should be broadly disallowed, why not something like social media too?

Legitimate question. Killing someone = bad. Poisoning people with whack medicine = bad. Causing loneliness and mental health epidemic = good ??

Societies have always decided what is and isn't allowed.

Well... we let people drink alcohol in excess, even though that often causes significant negative outcomes. We let people eat garbage overprocessed food, even though in quantity that can cause significant negative outcomes.

So we're not regulating every possible negative outcome. We're only blocking some that are "too" negative. Where's the line? It moves over time, and it changes from area to area. Food, for example, is regulated as to poison, but not as to just being junk.

We have the freedom of not living in a nanny state. Not everything that can cause harm is forbidden.