Mental health is an incredibly tricky thing to study.
Since all observations are of a person says they feel
and what observations can be made of the person and
what they may say various semi standard questionnaires.

The first problem is defining what depression is.
This is actually hard.

The present official definition can be found here:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6176119/

A great lecture on it by Professor Robert Sapolsky
can be found here:
https://www.youtube.com/watch?v=NOAgplgTxfc
He does this far far better than I ever can.
I include a tiny transcript below.

Even within the medical field the term is used
in ways that complicate matters.

We find
"Depression, otherwise known as major depressive disorder or clinical depression"

This is incorrect.

In everyday use the term depression is not equal to "clinical depression",
or "major depressive disorder"

In regular conversation depression is often used instead of "sad".
or "bummed".

If relying on a self-report survey that asks in part if you feel or have
felt depression over XXXX timeline a lot of people will say "yes".
Yet most will probably not meet the definition of major depressive disorder
or clinical depression.

If in everyday speech care was taken to only use depression for sad,
and clinical depression was used for the metal disorder then things
would be easier but as I showed above even medical professionals
tend to use the term "depression".

I think using the term "depression" has become a lot more popular than it was.

50 years ago, using it in common language was so common.

This gets better in the studies when (some) people attend college
where (some) people are seen by mental health professionals.
However, in my personal knowledge hte above is also primarily self-reported,
and not based on the treatment and therapy of the majority of the people in question.

Yet we have a non-representative sample.
Only people with privilege to attend high university and of them
the people who have sought out help.

So that is the problem of semantics.

A second problem is why
If a girl who spends only 1h a day on social media is less prone to "depression"
than a girl who spends 8h a day on social media does that mean social media is
the big factor.

I find it likely that it wold often be the case that a person with a healthy
social life, a good family, good activities, that takes up a lot of time.

While someone who spends 8h on social media per day perhaps needs more positive
activities.
Which can be incredibly difficult to achieve.

The argument has been made in different forms for a long time.
Too much TV, too much rock music, too much video games,
too much black metal.

Yet I do think that social media is far too often a destructive component
in someone's life. With severity than listening to a vinyl recording all day
or playing Quake. I would be quite happy if it all died.
It dont see that happening.

Partial transcript of a lecture by Professor Robert Sapolsky
""
So starting off, first giving a sense of symptoms. And right off the bat, we’ve got a sematic problem, which is we all use the word depression in an everyday sense. You get some bad news about something. You now have to replace the transmission in your car. Somebody disappoints you enormously. And you feel bummed. You feel depressed. You are down for a few days. That’s not the version of depression I’ll be talking about.

Next version, you do have some sort of large, legitimate loss, setback, whatever, losing a job, unemployment, death of a loved one. And you are extremely impaired by a sense of malaise for weeks afterward. And then you come out the other end. That’s sort of what I’ll be talking about.

But even more so what I’ll focus on is the subset of individuals who, when something like that occurs, falls into this depressive state. And weeks and months later, they still have not come out the other end.

Terminology. The everyday depression that we all have now and then, that sort of version. The second one, the something awful happens and you feel terrible for a while, and then come out the other end, a reactive depression. The third version, where you are flattened by it for long periods afterward, a major depression. And what you also see with people with major depression after a while is it doesn’t take something awful externally to trigger one of those again.