It's worth looking into the connection between social media use and mental illness, since you've raised an important point. The Economist article you cited raises important points about sadness, anxiety, and FOMO (fear of missing out), but I believe it's important to look more closely at the data supporting these assertions.
For example, research such as that conducted by Twenge et al. (2017) indicates a link between teenage mental health problems and more screen usage, although this link does not imply causality. Is it possible that social networking is a more common coping strategy among those who are already dealing with mental health issues?
The connection may also be affected by the kind of social media use. The impacts of passive scrolling may differ from those of active participation (such as posting and engaging). Studies by Orben and Przybylski (2019) highlight the importance of taking these subtleties into account.
To support your claim, have you thought about looking at research that uses meta-analyses or longitudinal data? These might give a more accurate picture of whether mental health issues are caused by excessive social media usage or by other things.