Not that it proves anything, but the converse argument, that depressed girls spend more time on social media or online in general, should also be examined. If the data could support this thesis to a similar degree, there is no conclusive statement about social media really inducing depression. It isn't just about effect size in my opinion but I don't see this problem answered.
That said, it doesn't need to be depression, there are other venues for negative influences. I am always ready to yell at the cloud for that.