I’m sure this still will come as a big surprise to many, but it’s pretty difficult for results to be as clear:

• Time spent using social media was not related to individual changes in depression or anxiety over 8 years.
• This lack of a relationship was found even in the transition between adolescence and emerging adulthood.
• Results were not stronger for girls or boys

Or a bit longer from the conclusion:

Specifically, there were no associations between time spent using social media and mental health across eight years, spanning early adolescence into young adulthood. That is, when individual adolescents used more social media than their own cross-time averages, they did not increase in either depression or anxiety. Similarly, decreases in an individual adolescents’ own time spent on social media did not indicate ensuing decreases in depression or anxiety. These findings are at odds with much of the research literature, but do seem to echo assertions from some cross-sectional studies suggesting that the reported link between social media and mental health might be somewhat exaggerated

Abstract of the study:

Many studies have found a link between time spent using social media and mental health issues, such as depression and anxiety. However, the existing research is plagued by cross-sectional research and lacks analytic techniques examining individual change over time. The current research involves an 8-year longitudinal study examining the association between time spent using social media and depression and anxiety at the intra-individual level. Participants included 500 adolescents who completed once-yearly questionnaires between the ages of 13 and 20. Results revealed that increased time spent on social media was not associated with increased mental health issues across development when examined at the individual level. Hopefully these results can move the field of research beyond its past focus on screen time.