So let's talk about what this study says and doesn't say. First off it's a quite high quality longitudinal study with 4 waves (different birth years) and sever

So let's talk about what this study says and doesn't say. First off it's a quite high quality longitudinal study with 4 waves (different birth years) and several thousand participants, where screen time was used to quantify social media use, and mental health outcomes were quantified through interviews with mental health professionals. It finds that "use of social media is not correlated with developing depression or anxiety". It also finds "developing depression or anxiety is not correlated with change in use of social media". It specifically does not say anything about whether using social media while having a mental health disorder is good or bad. Let's suppose you swap out "using social media" with "drinking alcohol", and "10-16 year olds" with "18-25 year olds". Nobody would be surprised if you found both a) that whether a person drinks alcohol or not isn't a good predictor for mental health issues, but also b) that if you have mental health issues, reducing alcohol consumption is very good for your mental health. Both of those can be true at the same time.