The study has a long way to go before it is completed, but there have been some preliminary findings. Journal NeuroImage recently published and article titled, "Screen media activity and brain structure in youth: Evidence for diverse structural correlation networks from the ABCD study." They have found evidence in their study for the maturational coupling hypothesis, which postulates that changes in coordinated patterns of changes in brain structure result in changes in human behavior. They have found a thinner cortex in children who spend a lot of time on electronic devices, such as smartphones and video games. It is not yet clear that what affect this early maturational change will have on these children. It is clear that a lot of daily screen time by children changes their brains. They have found that it is correlated with dysfunctional behavior in children, e.g., substance abuse and conduct disorder. They have also found that it is correlated with learning impairment.
Social media companies use cognitive science to design their products to prolong the user's time on their platform, which is pretty much saying that they are making their products to be be addictive. It may sound extreme to say that parents shouldn't allow their children to use these devices, but I think it is wise for parents to at least limit their children's use of electronic devices to an hour per day, and to abstain from use one day per week.