Put it this way: is it good for a child to spend an appreciable fraction of their day browsing social media? Did children previously just have free hours at hand to burn on this? The answer is of course no, there are not more hours in the day after the creation of social media, so its usage comes at the cost of something else in that child's life, usually their precious little downtime where they might plan and think about their own life. Or maybe at the cost of other activities that might be more engaging physically or mentally.