The nation’s top health official issued an extraordinary public warning on Tuesday about the risks of social media to young people, urging a push to fully understand the possible “harm to the mental health and well-being of children and adolescents.” In a 19-page advisory, the United States surgeon general, Dr. Vivek Murthy, noted that the effects of social media on adolescent mental health were not fully understood, and that social media can be beneficial to some users. Nonetheless, he wrote, “There are ample indicators that social media can also have a profound risk of harm to the mental health and...