The author mentions in his testimony, that the correlation between social media use and mental illness may be around r=0.10. This means in general terms that it it explains about 10% of of the impact. This leaves a lot of room for additional factors, and factors with much greater impact.
Actually it explains 1% of the impact, since the R-squared will be 0.01. But there's a huge amount of random noise in whether people get depressed, which we would always struggle to explain. A measure of the effect size is more interesting than a measure of R^2.
