Anecdotally, I was (as probably many here were) ahead of the curve in digital devices usage when I started hanging out in IRC and various web forums as a teenager in early 2000s. At some point I noticed that just using computers induced some amount of anxiety compared to "old tech"; whenever there was some period of time when I used my computer less (like christmas, vacations, etc.) and joined back to the "world of the normal people", I felt much calmer and happier. Even though I noticed this, it was difficult to log off during normal times since most of my life was in the internet.

Now everyone is using digital devices all the time and the "normal people world" has ceased to exist. Also almost everyone is anxious and/or depressed. I think this is not a coincidence. However, I do not think that this is due to social media per se, but using digital devices for anything (social media being just the reason why most people use them).

My theory is that just using digital devices for anything is somewhat stressful; you have to keep the eyes focused all the time (Can you think of other activities that require this? There aren't many and they are all somewhat stressful), you have to navigate all the various applications and menus, you have to occasionally solve minor problems that you run into when using the devices, etc.

Using digital devices is the same for your brain as heavy, repetitive physical labour is to your body; in small amounts it might even be healthy, but several hours every day is going to destroy your body/mind.