After acknowledging the problem, Facebook remained silent on the issue for nearly 24 hours before issuing an explanation and apology.
Facebook blamed a "server configuration change" Thursday for a massive outage affecting its applications around the world and brought fresh attention to the embattled social networking leader.
The outage affected users for at least 12 hours in most areas of the world, with the biggest impact in North America and Europe, according to the tracking website downdetector.com.
After acknowledging the problem Wednesday, Facebook remained silent on the issue for nearly 24 hours before issuing an explanation and apology around 1630 GMT Thursday.
"Yesterday, as a result of a server configuration change, many people had trouble accessing our apps and services," a Facebook tweet said.
"We've now resolved the issues and our systems are recovering. We're very sorry for the inconvenience and appreciate everyone's patience."
The outage was believed to be the worst ever for the internet giant that reaches an estimated 2.7 billion people with its core social network, Instagram and messaging applications.
As with previous disruptions, #FacebookDown quickly became a popular hashtag on rival platform Twitter.
On Wednesday, Facebook rejected speculation that the problem may have been the result of a denial-of-service attack -- a type of cyberattack that overloads the host of a service with superfluous requests.
A Facebook outage in November last year was attributed to a server problem, and a September disruption was said to have been the result of "networking issues."
Bloomberg News reported that Facebook was considering refunds for advertisers whose messages could not be delivered, although the company did not immediately respond to AFP's request for confirmation.
Reshuffling amid turmoil
The outage came amid a period of turmoil for Facebook, which appeared to intensify Thursday with the announcement of the departure of two senior executives at the social network.
Chief product officer Chris Cox announced his resignation, becoming the highest-ranking manager to depart amid the turmoil at the leading social network.
Cox made his announcement on his Facebook page, saying he was leaving "with great sadness" after 13 years.
While Cox gave no specific reason for his move, he noted that Facebook chief Mark Zuckerberg has unveiled a new direction away from being the "digital town square" to focus on smaller-scale, private interactions.
"As Mark has outlined, we are turning a new page in our product direction, focused on an encrypted, interoperable, messaging network," Cox wrote.
Facebook also said WhatsApp chief Chris Daniels is leaving the company.
The new head of the Facebook-owned application will be Fidji Simo, who was previously in charge of video, games and monetization at Facebook. Will Cathcart will be the new head of WhatsApp.
The New York Times reported late Wednesday that US prosecutors had launched a criminal investigation into the social network's practice of sharing users' data with companies without letting them know.
According to the report, a grand jury in New York has subpoenaed information from at least two major smartphone makers about such arrangements with Facebook.
The news comes with regulators, investigators and elected officials in the US and elsewhere in the world digging into the data sharing practices of Facebook.
The social network's handling of user data has been a flashpoint for controversy since it admitted last year that Cambridge Analytica, a political consultancy which did work for Donald Trump's 2016 election campaign, used an app that may have hijacked the private details of 87 million users.
Facebook suggested there was nothing new in the New York Times report.
"It has already been reported that there are ongoing federal investigations, including by the Department of Justice," a spokesman told AFP.
"As we've said before, 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."
Facebook has maintained that it shared limited amounts of user data with smartphone makers and other outside partners to enable its services to work well on devices or with applications.
Regulators, and now prosecutors, appear intent on determining whether this was done in ways that let users know what was happening and protected privacy.
Over the past year, the social network has announced a series of moves to tighten handling of data, including eliminating most of its data-sharing partnerships with outside companies.