Underage drinking is still more common than it should be,

Sure, but it's significantly lower than legal drinking.

We as a society acknowledge the harm of underage drinking so prohibition is effective. Prohibition of adult drinking was puritan bullshit the majority didn't agree with so it didn't work.

I think you'd find a majority of parents agree social media is shit, but they're unwilling to isolate their child. In this case prohibition would be effective.