Is This Facebook’s ‘Big Tobacco’ Moment?

Facebook Inc. executives have long boasted that its platforms are safe, even as they invested in ways to keep teenagers hooked and hid what they knew about the side effects. Sound familiar? Critics say Big Tobacco once used the same playbook, and it’s fueling a whole new level of outrage against the social media giant.

Facebook consistently played down its own research that showed how photo-sharing app Instagram can harm the mental well-being of its youngest users, according to a report in the Wall Street Journal. Almost a third of young teen girls told...