More than 7.5 million Facebook users are below the site's minimum age of 13, including five million below the age of 10, according to Consumer Reports.
The report claims that most child users under the age of 10 are largely unsupervised in their use of the site and that more than one million children have been harassed, threatened or bullied on the site in the past year. Parents of 13- and 14-year-olds were much more likely to monitor their usage of the site.
Facebook asks users for their birth date during the sign-up process and rejects who that fall below the minimum age limit, but an underage user can get around the limit by entering a false birth date.
"Recent reports have highlighted just how difficult it is to implement age restrictions on the Internet and that there is no single solution to ensuring younger children don’t circumvent a system or lie about their age," said Facebook in an emailed response to a request for comment.
"We appreciate the attention that these reports and other experts are giving this matter and believe this will provide an opportunity for parents, teachers, safety advocates and Internet services to focus on this area, with the ultimate goal of keeping young people of all ages safe online."
Facebook does provide additional safety and privacy controls for users below the age of 18, and the firm deletes the accounts of underage users when it identifies them.