Millions of Facebook users endured several hours without the social media giant's family of networking apps on Wednesday and Thursday after the company experienced it most severe loss of service to date.
The California-based enterprise, which has 2.3 billion regular users globally, said on Thursday it was recovering from the disruption, which lasted for more than 14 hours.
Facebook's main social networking site, its two messaging apps, and the picture-sharing site Instagram were all impacted.
The BBC noted Facebook had a similar outage back in 2008 but, with only 150 million regular users at the time, the latest disruption was clearly more serious.
Facebook posted a statement on Twitter during the outage saying: "We're aware that some people are currently having trouble accessing the Facebook family of apps. We're working to resolve the issue as soon as possible."
Sky News said some users got a message saying the site was down for "required maintenance".
Instagram was the first of the company's platforms to bounce back, announcing on Thursday morning, UK time, it was working again.
The company did not immediately go into details about what had caused the problem that hit Facebook, Instagram, WhatsApp and Messenger but insisted it was not a cyberattack. NBC said on Thursday a Facebook source had blamed a "database overload".
DownDetector, which monitors the availability of apps globally, said the problems impacted people everywhere but The Guardian said they were at their worst in the Americas and Europe.
Bloomberg said Facebook, which gets most of its revenue from advertising, was investigating "the possibility of refunds for advertisers".
Google also had problems on Wednesday. Users reported several hours of disruption to Gmail, Google Drive, Hangouts, and Google Maps.
The outages came at an interesting time for the corporations, with legislators in the US considering the merits of breaking up large technology companies because of their perceived political power and the spread of so-called fake news, as well as the lack of choice among users that means large numbers of people could be impacted by service disruptions. That argument is likely to now gain further traction.
The New York Times, meanwhile, said on Thursday US federal prosecutors were investigating Facebook over its alleged data-sharing deals with other companies.
The paper said a grand jury in New York had subpoenaed records from "at least two prominent makers of smartphones and other devices" with a view to find out whether Facebook had sold users' personal information.
A Facebook spokesman told the paper: "We are cooperating with investigators and take those probes seriously. We've provided public testimony, answered questions, and pledged that we will continue to do so."
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