kids need to take to learn control of things that are harmful and how to engage them rather than be kept away from them. This avoidance behavior to me seems to be a consequence of now endemic helicopter parenting. If you hide things from kids, they don't gain the confidence of dealing with them or coming to you on their own terms.
The goal shouldn't be to reduce social media usage, it should be to equip teenagers with tools to be resilient and take control of the way they use social media. It's in a sense like bullying. Yes, you can shield your kids from exposure to it, or you can teach them to confront it. What's better long term?
And that's the thing: as a teenager if you don't have access to <disallowed thing>, there's surely at least one kid in your class, whose parents are either largely absent or particularly lenient, and who will provide you with whatever you need.
In the case of social media I'd imagine a secret separate phone without parental restrictions and SIM card swapped out for the time being. Maybe even a separate SIM card, should the first one be restricted on the provider level. Or just rooting the device and rendering any control moot. There's always this one guy who'll help you out if your parents are too strict in your view.
Of course there's going to be some reduction, but you have to ask yourself: what's the plan when the kid turns eighteen and you can't legally control their internet usage - not when they have a SIM card on their name? You just let go of any restrictions on that day?
No argument that education is important, and that some would seek and find a way around the restriction, like they do with alcohol. But I’m sure that restricting access to social media under a certain age like it is the case for alcohol would significantly reduce the use of social media in the affected age group.