While this fact check holds promise, I think it could use a little work. While you clearly did research and included two reputable sources, I think saying they coincide totally to explain how social media is related to mental illness is slightly misleading. The article from the Pew research center says nothing about mental health at all, it more so splits up the increase in social media use by demographic, and while the Stanford article does say there is a connection between social media use and mental health issues, it says this directly folliwng that statement, :"The associations, though not by itself proof of causation, at least some reason for concern."
I think that sums up the proof you presented for this factcheck. It cannot be used as direct evidence, or at least, it is not strong by itself. While I think your sources are goos and are almost there, seeing as they relate strongly to the issue at hand, they do not quite tie together the threads the claim of social media causing a decline in mental health is trying to. Another article or study having data with a stronger correlation may help this fact-check.