The harm social media causes primarily due to two things. First is the lack of in-person contact, something children always experienced prior to the proliferation of the smartphone. Of course, there is always ups and downs, but it is real. Children don't learn the subtlety of human contact, and although it too has its pros and cons, it is an interaction that cannot be duplicated on video calls.
Second is the rampant misinformation and disinformation over the Internet. Enforced by the social network's algorithms that push similar content to the user over and over again, very little "two sides to a story" is ever heard. Debate is rare; in fact, there is insufficient debate in human gatherings, although in-person interaction enables people to be more empathetic and compassionate. See
social media and
disinformation.