> Correlation is not causation, and one area of further study is whether greater social media usage leads to poor mental health outcomes or whether those who are depressed and unhappy are drawn to spend more time on social media.
These are correlational studies. They aren't saying that they have evidence that Facebook causes depression. They're saying that increased social media usage is correlated with depression.
Stepping back, it appears that screen time is correlated with depressive symptoms, with or without social media. One such study: Association between screen time and depression among US adults https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5574844/ This includes watching TV or simply using a computer for leisure of any sort.
I know Facebook is a popular villain, but we shouldn't be jumping to conclusions that this data means Facebook causes depression or that Facebook is uniquely worse than other forms of screen time in its correlation with depression.