Facebook must disclose any available records on the number of underage account holders in Northern Ireland, the High Court in Belfast has ruled.
It ruled any existing records should be supplied for a lawsuit that involves a vulnerable girl who contacted men and posted sexual photos on the website.
Facebook is being sued by the girl's father, for alleged negligence and breaching her right to privacy.
Under Facebook's own policy, no-one under 13 should set up an account.
But with the schoolgirl now under a care order, lawyers for her father claimed an open registration system meant it was too easy for her to set up profiles and be at potential risk from paedophiles.
From the age of 11, she created four different accounts to publish sexually suggestive and inappropriate photos, the court heard.
She received text messages with extreme sexual content from men as a result of her personal details appearing on Facebook, it is alleged.
However, her accounts were deactivated as soon as reports were received by the company.
As part of the ongoing legal action, attempts were made to secure more details about the number of underage users reported and identified.
In a detailed ruling, the judge blocked many other requests for more information from Facebook.
But he held that specific discovery should be made on any documentation containing notes and records that company holds on use of its network by children under 13 in Northern Ireland or the UK as a whole, between 2011 and 2014.
"If the defendants do have them, they should be discovered," the judge said.
"If they do not have them, then obviously they can properly indicate that they do not have such information in their possession, custody or power."
The same ruling was made on a request for details on the number of account holders in Northern Ireland generally.
According to the judge, that information appeared relevant to assessing the size of the task confronting Facebook "if taking steps to address the mischief of underage children registering".
The court also heard the company does not retain data on reports of underage users for more than six months.
Countering the claim that no more statistics were available, a lawyer for the girl's father cited an alleged statement by its chief privacy advisor in 2011 that 20,000 people a day are removed from the site for being underage.
Despite recognising the "factual standoff" on the issue, the judge refused to order Facebook to comply with that request, on the basis that it does not have the information.
He added: "If it should emerge that the defendant's assertions are unsustainable, then at the very least the case may be adjourned in order to compel appropriate replies to the interrogatories with attendant cost consequences."