Here's the paper: https://epi.org.uk/wp-content/uploads/2021/01/EPI-PT_Young-p... It's all correlational (observing that people with higher social media use are worse off in various ways). This approach is not capable of detecting reverse causation (people use social media more because they are unhappy) or third causes (something else causes people to both use more social media and also be unhappy)
The news of the world has been pretty bad all around, getting worse and worse over the past decade. I can't help but wonder if being online plugs someone more deeply into that news, and the actual cause of the damage to one's mental health is more exposure to multi-sourced narratives unfiltered by the editorial voice often employed by traditional news media.
What if these young people are showing signs of mental trauma because mental trauma is a predictable reaction to being informed about the state of the world, and being more online leaves one more informed?