Facebook should have stopped a Northern Ireland schoolgirl from setting up accounts to post sexual photos and message men, the High Court heard today.
Lawyers for the girl's father claimed the social networking giant acted negligently and misused her private information.
The allegations were made amid legal attempts to secure more information for a lawsuit against the company.
Proceedings have been issued against Facebook and the health trust with responsibility for looking after the girl who was subject to a care order.
Under Facebook's policy no-one under 13 is allowed to be a user.
But it is alleged that an open registration system enables children to log-in and potentially puts them at risk from paedophiles.
The girl posted sexually suggestive images of herself on the site between the ages of 11 and 13, the court heard.
She has used different accounts and been in exchanges with older men, including one who is subject to a restraining order.
During a preliminary hearing today Edward Fitzgerald QC, for the girl's father, said: "There was concern that she was out of control, placing herself at severe risk of harm and at times suicidal.
"Our case is that Facebook should not have permitted her to set up and operate this account given her extreme youth."
Facebook's defence to the action includes a denial that an open registration method is in operation.
But Mr Fitzgerald claimed even the company's founder and chief executive, Mark Zuckerberg, has acknowledged such a system is used.
He referred Mr Justice Gillen to a newspaper article where another Facebook executive is quoted as saying there is almost nothing it can do to stop young users setting up profiles.
Academic research suggests more than a third of UK 9-12 year olds in the UK now have their own page on the social network, the court heard.
Mr Fitzgerald contended that the defendant was aware a significant number of profile account holders were younger than 13.
"That is denied, and we say that denial is essentially incredible," he said.
The barrister confirmed he was seeking information on how many times Facebook has had to delete profiles and what monitoring has been carried out.
The hearing continues.