Hamish MacEwan

The phenomenon has sparked concern among parents and child safety advocates who fear that young girls are making themselves vulnerable to adult strangers and participating in often cruel social interactions at a sensitive period of development.

But the contests are the latest example of how technology is pervading the lives of children in ways that parents and teachers struggle to understand or monitor.

“What started out as just a photo-sharing site has become something really pernicious for young girls,” said Rachel Simmons, author of Odd Girl Out and a speaker on youth and girls.