On photo-sharing site Instagram, search the keywords #Fairfax, #Rockville or #DC and up pops hundreds of photos from children. Among them, until recently, were many from 12-year-old Kyle. His full name, his middle school in Gaithersburg, Md., and his favorite Montgomery County hangouts were on public display before his parents put a stop to it.

Technically, Kyle wasn’t supposed to be on Instagram, the mobile app owned by Facebook. The company’s policy sets the minimum age at 13. But Kyle said he was able to join easily, no questions asked. Within minutes of setting up his account last fall, he was uploading “selfies” of his cherubic face and blond mop top, and tagging photos of friends with their names.

Kyle is among the underage users flocking to Instagram, a trend that’s creating a new social problem for Facebook.

The photo-sharing service is seeing tremendous growth, doubling in size to 100 million users in about a year. But child advocates and some parents say too much of its rise has been driven by preteens or even younger kids.

These advocates say they worry about whether Instagram is collecting the personal information of young children — and whether the company is doing enough to make sure kids are safe from adult strangers.

Over the past two weeks, more than 4,500 people signed a petition on Change.org that calls for Facebook to automatically set the accounts of children and teens to private. It also asks the company to disable GPS technology that can pinpoint where children take photos.

“Facebook is not doing enough to ensure children under 13 don’t have access to the site,” said Joy Spencer, a director of child safety for public interest group the Center for Digital Democracy. “That raises a number of concerns about safety, and because Instagram then is able to collect personally identifiable information on children, which can be used to target ads toward them in the future.”

New research shows Instagram has become particularly popular among youths. But the service doesn’t make even a nominal effort to keep young children from signing up, privacy advocates said. Anyone can register with a fake name. The app does not ask for any personal information.

Under a revision of the federal government’s child privacy law, which is set to take effect July 1, social networks and other websites must get a parent’s consent if they collect personal information such as photos, email addresses or videos from users younger than 13.

Instagram said it doesn’t track how many underage users are on its service, which primarily serves mobile phone or tablet owners. When asked why, spokeswoman Nicky Jackson Colaco said, “Like many other platforms, Instagram only asks for data that is essential to operate our service.”

Colaco said Instagram is trying to make the app safer.

Other popular social media companies, such as Tumblr, Snapchat and Twitter, have similar policies banning preteens, but few are successful in keeping young children off their services, privacy advocates say. Photo-sharing sites pose particular risks to minors, they say.

Paul Miller, like many parents, helped his daughters, 11 and 13, create accounts. The tech and media consultant, who is based in San Francisco, wasn’t aware of the age limit.

He quickly learned his children needed a basic primer on etiquette and behavior. No posting photos that could make others feel excluded. No nasty comments. Say “no” to strangers.

He said he hopes Instagram will take a more active role in educating kids on best practices.

“It’s a tender age, and the social etiquette isn’t there for this technology,” he said.