Another approach is longitudinal, measuring narcissism and social media use over time and seeing if they are mutually reinforcing; that is, whether narcissism predicts increased social media use and whether that, in turn, predicts increasing narcissism. At least one study shows this pattern.
It might also be the case that social media inflates the narcissism of those already predisposed, but has no effect on others.
So it is plausible that social media use increases narcissism. But there is also longitudinal research suggesting that social media use can make children more empathetic. For example, children who spend time engaged with their friends on social media might become more concerned with the up and downs in their friends’ lives.