I don't think it even has anything to do with interaction increasing. They told people to essentially fast a bit from an indulgence (the widely held assumption is that it is "good" not to be too attached to screens, which these college kids would have been told since childhood screen time limits imposed by their parents) and gave them a feedback method to perform their commitment and virtuousness (rather than having to chronicle their continued indulgence like the control).

I think a control group reporting they ate green vegetables once a day or drank water instead of a soft drink might have shown the same positive mental health effect, and the study may not say anything about social media as such at all.