Herein lies the rub. How do you decide what the threshold is? Who gets to decide what that threshold is, and how do you do it without inviting accusations of regulatory capture?
If you make it blanket all social networks, then things like discord and even public slack orgs will inadvertently become collateral damage. If you make it focussed on only a few large ones, e.g. Twitter, Facebook, Instagram and TikTok, then something else will pop up to take it's place. It'll become a game of whack-a-mole. Users are supposedly already migrating in droves to some other TikTok clone.
I'm not really sure what the solution is though. Regulate the shit out of it to the extent where it becomes a government-provided utility or something?
The reality is people want social media because they are addicted to it. Getting rid of social media will be like the war on drugs: completely ineffective. The danger here is that the drug is very easy to create, impossible to control and extremely lucrative.