Facebook is under criminal investigation for a series of controversial data-sharing deals it signed with other major tech companies, according to reports.
US government prosecutors are looking into how Facebook shared personal information from hundreds of millions of its users with more than 150 firms, including Amazon, Apple, Microsoft and Sony, sometimes without their consent.
A grand jury in New York has ordered two smartphone makers which were party to such deals to hand over documents, according to the New York Times, though it is not known what exactly the jury is investigating.
The probe constitutes a sharp escalation of Facebook's legal problems in the USA, where it is under scrutiny from four separate government agencies, including the US justice department, as well as from legislators.
“We are cooperating with investigators and take those probes seriously,” said a company spokesman. “We’ve provided public testimony, answered questions and pledged that we will continue to do so.”
Facebook is already entangled in a criminal investigation into the Cambridge Analytica scandal, in which political consultants used its systems to improperly harvest data from 87 million people.
But the social network has always said that it was misled by Cambridge Analytica, describing its role in the scandal as having been too lax in policing how other companies were using its service.
By contrast, this investigation appears to concern routine aspects of Facebook's own business, which it described last December as public, widely scrutinised and innocuous.
The deals, previously reported by the Wall Street Journal and the New York Times, involved Facebook sharing its users' private data with other companies on a scale far beyond what it had publicly admitted.
The social network allowed Sony, Amazon and Microsoft to get email address from all a Facebook user's friends, even if the friends themselves had not agreed to share them – a form of data sharing exploited by Cambridge Analytica, which Facebook had previously said it had ended in 2015.
Apple was reportedly allowed to access contact numbers for Facebook users who had explicitly disabled all sharing with outside entities, while Microsoft's search engine, Bing, was allowed to see the names of users' friends.
Many of the app features which these agreements were designed to support were discontinued years ago, and many of the companies claimed not to know they had such data, raising the question of how well Facebook had kept track of it.
Konstantinos Papamiltiadis, Facebook's director of developer platforms and programs, defended the deals, saying they were necessary to integrate Facebook's services with various smartphones and apps such as Spotify and Netflix.
"People want to use Facebook features on a variety of devices and products, many of which we don’t support ourselves," he said. "Across the industry, companies like Facebook partnered with other companies to build integrations.
"To be clear, none of these partnerships or features gave companies access to information without people’s permission."
But he admitted that Facebook should not have continued to share data after shutting down the features for which the data was intended, and that the company needed "tighter management" over how other companies could access its services.
Facebook is reportedly locked in negotiations with the US Federal Trade Commission over claims that it violated a privacy settlement made in 2011. If the FTC upholds those claims, it could result in a multi-billion dollar fine, which would be the largest in the agency's history.
Facebook contends that it has not violated that agreement, and that its data sharing deals were kosher because the companies in question were merely processing information on behalf of Facebook.
The New York Times' report came after a day of chaotic glitches and outages which rendered Facebook and its sister apps Instagram WhatsApp all but inaccessible in many regions of the world.
Mr Zuckerberg could also face a legal summons from the Canadian parliament if he refuses to appear before an international hearing in Ottowa in May.