Some 7.5 million of the 20 million minors who used Facebook in the past year were younger than 13, and a million of them were bullied, harassed or threatened on the site, a study released on Tuesday said.
More than five million Facebook users were 10 years old or younger, and they were allowed to use Facebook largely without parental supervision, leaving them vulnerable to threats ranging from malware to identity theft to sexualpredators, the State of the Net survey by Consumer Reports said.
Facebook's terms of service require users to be at least 13 years old but many children, or their parents, get around that rule by giving a false birth date when they sign up for the social networking site.
Parents of kids 10 and younger who use Facebook "seem to be largely unconcerned" by their children's use of the site, possibly because they think a young child is less vulnerable to Internet risks, the study said.
But while a 10-year-old might not download pornography on the Internet, he or she does "need protection from other hazards that might lurk on the Internet, such as links that infect their computer with malware and invitations from strangers, not to mention bullies," the study said.
More than five million US households have been exposed in the past year to "some type of abuse" via Facebook, including virus infections, identity theft and bullying, said the study, for which 2,089 US households were interviewed earlier this year.
A separate study published last month by Richard Power of Carnegie Mellon University's CyLab cybersecurity research center, found that 10 percent of children in the United States have had their social security numbers stolen and used to apply for driver's licenses and loans to buy cars or even houses.
The rate of identity theft among children was 51 times higher than for adults, Power said in his study.
"The online experience has changed childhood, for both better and worse.
"It enables children to explore the life of the world, but without proper precautions, it also enables the world to explore your child's life," he said.
A news story quoted in the CyLab study said a group of teens was contacted on Facebook by a fake talent scout "promising them a career as a model in exchange for photos of themselves in lingerie."
Consumer Reports urged parents to delete their pre-teens' Facebook accounts -- or ask Facebook to do so by using the site's "report an underage child" form -- and to monitor teenage kids' accounts by friending them or keeping an eye on their activity via siblings' or friends' Facebookpages.
It also called on Facebook to "beef up its screening to drastically reduce the number of underage members."
Facebook spokesman Andrew Noyes said in a statement sent to AFP that the social networking site encourages "communication between parents/guardians and kids about their use of the Internet.
"Just as parents are always teaching and reminding kids how to cross the road safely, talking about Internet safety should be just as important a lesson to learn," Noyes said.
But he also stressed, "Just how difficult it is to implement age restrictions on the Internet" and said there is "no single solution to ensuring younger children don't circumvent a system or lie about their age."