This is absolutely a myth! Here's a collection of papers showing the reality, which is that any negative effect from social media use is tiny (for both teenagers and adults) and a small number of overall users, and far more people get a positive effect: bsky.app/profile/raha...
rahaeliI am tired of pasting dozens of links at people, so I've loaded a bunch of the papers into Paperpile and exported a bibliography. You can find a collection of 34 papers on social media's effects on teenagers (and sometimes adults) and alternate hypotheses here: paperpile.com/shared/smMqS...