We aren't concerned only about existing addicts, but potential future addicts. Especially for something like social media with strong network effects, where decreasing use is non-linear.

Banning substances dramatically decreases use: https://link.springer.com/article/10.1007/s00414-015-1184-4 and legalized opioids dramatically increased heroin use: https://www.tandfonline.com/doi/abs/10.3109/15360288.2015.10... Access matters.

The question is always: A. What do people use instead? (banning pot, for example, increases use of heroin and alcohol, which is good for alcohol companies but bad for public health. If banning social media sent kids to 24/7 news channels, it might not help, but I haven't seen evidence of that.) B. How much is organized crime funded by the increased black market? (In this case, kids are a limited population that doesn't have a lot of money, so the answer is probably "not much".)

Banning substances naturally decreases use, that's obvious, but prohibition criminalizes use, which will always persist. You cannot stop drug use. Drug legalization so far has resulted in declining use of dangerous substances like tobacco and alcohol. Far more young people today choose smoking pot over smoking cigarettes and drinking alcohol. Many people choose not to drink because they've observed the widespread dangerous effects of it since it's been legal. If heroin was made legal all over the world today, you'd 100% see increased use. But maybe humanity needs to see the consequences in order to respect them? Just like alcohol and cigarettes?