You're missing an important distinction that I may not have made clear: a lot of activities that can be harmful to children can also be valuable. There's a lot of harmless fun on television, walking alone to the park or a neighbor's house fosters a healthy sense of independence, weight training done safely can help build strength and confidence and sugary caffeinated sodas in moderation are… tasty? (Not sure about the benefit on that last one.)

Like these activities, social media can have their benefits even for young people: contact with friends and family far away, connection with others with similar interests, and a broader view of the world, among others. With guidance, kids can experience those benefits without too much of the harm.

In most of the cases you point out — drinking, driving, buying firearms, and sharing pornographic pictures of themselves — any good there may be is vastly outweighed by the dangers. I don't believe the harm so disproportionate with social media, but I suppose others might disagree.