[myrmidon]

I would argue that negative effects from social media must be expected to be much harder to detect, because they would/could only manifest in changed behavior (possibly long-term!), and given how pervasive social media has become there is not even a good baseline for studies.

Picture e.g. social media exposure having a long-term negative effect on ability to focus, motivation to learn/work/self-improve and general satisfaction/self-image/outlook-- these are all somewhat plausible, and could be extremely difficult to isolate in studies.

Regarding smoking in particular: Imagine there being absolutely zero cancer/physical health effects, everything else being exactly equal. Would that make it okay to you? Because to me, it absolutely would NOT-- a society that allows under-age persons to become strongly addicted to something, handing corporations a convenient metaphorical straw to suck money out of them for the rest of their lives-- that is a society that has failed their offspring, to me. And social media (give its addictiveness and value extraction via ads etc.), is basically in exactly that situation already.