Instagram 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.
The photo-sharing site has become wildly popular as a way to trade pictures of pets and friends.
But a new trend on the site is making parents cringe - beauty pageants, in which thousands of young girls, many appearing no older than 12 or 13, submit photographs of themselves for others to judge, the Washington Post reported.
Any of Instagram's 30 million users can vote on the appearance of the girls in a comments section of the post.
Once a girl's photo receives a certain number of negative remarks, the pageant host, who can remain anonymous, can update it with a big red X or the word "OUT" scratched across her face.
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.
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," Rachel Simmons, author of "Odd Girl Out" and a speaker on youth and girls said.
"What happened was, like most social media experiences, girls co-opted it and imposed their social life on it to compete for attention and in a very exaggerated way," she said.
Contests are also appearing on other social media sites, including Tumblr and Snapchat - mobile apps that have grown in popularity among youth.