Facebook has 7.5 million users who are underage and are "not supposed" to be using the social networking site, according to a new report.

Of the 20 million minors who actively used Facebook in the past year, more than one-third or about 7.5 million were younger than 13 and not supposed to be able to use the site, Consumer Reports, an American consumer advocacy publication, said in a study.

Among the young users, more than five million were 10 years and under, whose accounts were largely unsupervised by their parents. One million children were harassed, threatened and subjected to cyberbullying on the site in the past year.

The study said majority of parents of 10-year olds and younger kids were "largely unconcerned" about their child using the site.

"Despite Facebook's age requirements, many kids are using the site who should not be," Consumer Reports technology editor Jeff Fox said in the statement.

"What is even more troubling was that a majority of parents of kids 10 and under seemed largely unconcerned by their children's use of the site."

Facebook, which has over 500 million users, screens applicants by asking for their birth date and rejecting those too young.

However, a pre-teenager can join the network by falsifying his or her birth date. Further, since joining Facebook does not require a credit card, it is easier to give incorrect information to join the network.

The report said parents of 10 year olds and younger kids on Facebook seem to be largely unconcerned. Only 18 per cent made their child a Facebook friend, which is the best way to monitor the child. Only 10 per cent of parents of kids 10 and under had frank talks about appropriate online behaviour and threats.

"Parents think their young children are less likely to take risks online and their younger kids are not interested in porn. With a 10-year-old mentality, they are only interested in 10-year-old things," says Vanessa Van Petten, creator of blog Radical Parenting. .

But those parents would be mistaken. Ten-year-olds need protection from other hazards that might lurk on the Internet, such as links that infect their computer with malware and invitations from strangers, as well as bullies.

The report said parents who object to their teenager using Facebook cannot count on any help from the site. Citing federal electronic privacy laws, Facebook does not close a teenager's account just because a parent requests it.

Consumer Reports suggested that parents should either delete their pre-teenager's Facebook account or ask Facebook to do so by using its "report an underage child" form. Parents could monitor activities of the children by joining their circle of Facebook friends.

About 18 per cent parents kept tabs on their 13-17 year olds through their friends or siblings.

Parents should also get to know the technology and should "use Facebook yourself, so you are savvy about the privacy issues," says Denise Terry, chief 'safety mom' at SafetyWeb, an Internet-monitoring service for parents.

The report said, Facebook on its part, should beef up its screening to drastically reduce the number of underage members and make its privacy controls even more accessible.

In April, Facebook had rolled out new security tools aimed at improving how users report bullying, fake profiles and offensive content, in addition to announcing a teacher's guide to the social network.