About a year ago, teenager Anastasia Vlasova started seeing a therapist. She had developed an eating disorder, and had a clear idea of what led to it: her time on Instagram.

She joined the platform at 13, and eventually was spending three hours a day entranced by the seemingly perfect lives and bodies of the fitness influencers who posted on the app.

“When I went on Instagram, all I saw were images of chiseled bodies, perfect abs and women doing 100 burpees in 10 minutes,” said Ms. Vlasova, now 18, who lives in Reston, Va.

Around that time, researchers inside Instagram, which is owned by Facebook Inc., were studying this kind of experience and asking whether it was part of a broader phenomenon. Their findings confirmed some serious problems.