US senators on Thursday grilled Facebook Inc on its plans to better protect young users on its apps, drawing on leaked internal research that showed the social media giant was aware of how its Instagram app harmed the mental health of teens.

The hearing in front of the Senate consumer protection subcommittee was called after the Wall Street Journal published several stories earlier this month about how Facebook knew Instagram caused some, teen girls in particular, to feel badly about their self-image. After growing opposition to the project, Facebook put plans for Instagram Kids, aimed at pre-teens, on hold this week.

Antigone Davis, Facebook's global head of safety...