This article explains that there is no good scientific evidence to prove the effects of social media use on mental health in adults, and it clarifies further that no studies have been done on children or teens: There is insufficient scientific data to support social media as either good or bad for mental health.

This does not mean that social media has a neutral effect on mental health. It means that we haven’t collected enough data to know scientifically anything about social media’s effect on mental health. To me this means we just need bigger, better randomized controlled studies (practically impossible) if we actually want to answer the question scientifically. Alternatively, observational studies of societies undergoing legislative reform could shed some less perfect light.

Meanwhile, there are these expert perspectives on smartphones and kids in general: See Episode 6 of https://www.thefp.com/raisingparents-emilyoster