The outage of Facebook, Facebook Messenger, Instagram, and WhatsApp on Monday was one of the longest in Facebook history, stranding billions of people who rely on the social media giant and its applications for everything from communicating with friends to operating companies and checking into websites.

According to Downdetector.com, the social network and Facebook-owned sites went down about 11:30 a.m. EDT Monday. Some users were able to access the platforms at 5:40 p.m., however not all functionalities were restored.

Facebook stated that no user data has been stolen across all of its platforms.

Facebook would not specify what caused the outage in a statement issued to USA TODAY: "We apologize to everyone who was inconvenienced by today's disruptions on our platforms. We understand that billions of individuals and companies all over the world rely on our goods and services to stay connected. We appreciate your patience while we return to service."

Mike Schroepfer, Facebook's Chief Technology Officer, apologized once more to Twitter users: "Facebook services are currently operational again, however it may take some time to reach 100%. I apologize to every little and large business, family, and individual that relies on us."

The disruption impacted Facebook's main source of revenue — advertisements. According to eMarketer, Facebook spends more than $48 billion on digital advertising in the United States each year.

That's why Facebook raced to reactivate the sites. The business stated late Monday that the fundamental reason of the outage affected several of its internal systems, making diagnosis and resolution more difficult.

According to Facebook's experts, the incident was caused by a networking fault that disrupted connections between its data centers. With the servers unable to communicate, the difficulties grew, triggering outages across its networks and its three key social platforms, which are now being restored - slowly, according to Facebook.

According to Luke Deryckx, CTO of internet testing business Ookla, which owns the online monitoring site, the global Facebook outage evolved to be one of the greatest Downdetector has monitored in terms of complaints and length.

"Because of the combined popularity of Facebook, Instagram, WhatsApp, and Facebook Messenger, today's service outage has impacted billions of people. When Facebook is down, the internet has a horrible day, and today is no exception."

Facebook tweeted around an hour into the outage, "We are aware that some users are having difficulty accessing our applications and products. We are working hard to restore normalcy as soon as possible and apologies for any inconvenience.

The outage occurred after a whistleblower claimed that the world's largest social network prioritized money over the safety of its members.

Former Facebook product manager Frances Haugen claimed in an exclusive “60 Minutes” interview Sunday on CBS that a change in the information flow in Facebook's news feed in 2018 resulted to increased division and ill will in a network purportedly designed to bring people together.

Facebook's shares dropped over 5% on Monday.

How did all of these networks become compromised at the same time? Brian Krebs, a computer security journalist, tweeted a proposed solution to some of the issues: Facebook and Instagram were reportedly deleted from the DNS (Domain Name System) servers that act as the internet's white pages.

"The DNS entries that inform computers how to access Facebook.com or Instagram.com were removed from the global routing tables this morning," Krebs tweeted. "We have no idea why this modification was made. It might have been the result of a faulty internal, system-wide modification or upgrade. At this time, it's just speculation as to why. Facebook is the only owner of its DNS records."

According to web infrastructure provider Cloudflare, Facebook's service went inaccessible – and remained down Monday afternoon – as well as unreachable on the internet.

This appears to have happened as a result of an issue in the Border Gateway Protocol (BGP), an internet traffic standard. "From what we understand of the real issue, it is a worldwide BGP configuration issue," said Usman Muzaffar, Cloudflare's senior vice president of engineering. "In our experience, they are generally errors, not assaults."

When traffic is sent to Facebook's internet addresses, the addresses are essentially non-existent due to the problem. "Visitors attempting to access a Facebook property, such as facebook.com, will not receive a response, and so the page will not load," Muzaffar said.

"It's 100 percent an internet routing issue," said BlueCat Networks' chief strategy officer, Andrew Wertkin. "The routes are not available on the internet. We don't know why this happened or what caused it. The routes were pulled or discontinued. We don't know why they were removed."

Is Facebook.com available for purchase? Twitter responds

During the course of the event, cybersecurity expert Anis Haboubi posted what seems to be a "For Sale" listing for the Facebook.com domain.

Krebs saw it as well, and discovered that domain name provider GoDaddy was also selling Facebook.com. "Bidding starts at.... one beeleon dollar!" he said on Twitter.

Facebook.com was never seriously considered for sale, according to online domain firm GoDaddy.com in a statement to USA TODAY: "A third party who does not own Facebook.com attempted to offer it for sale on Uniregistry.com, and we featured it in search results unintentionally. Because the third party did not own or control the domain, it was never in danger of being sold and is still in the hands of its present owner. The listing has been deleted and has nothing to do with any platform difficulties that Facebook may be experiencing.”

A rumor arose on social media that someone was selling scraped data from 1.5 billion Facebook users on a hacker site. Researchers advised caution, stating that the material originated from a 2-week-old thread and that they were unclear if the data was genuine. Someone in the discussion said they paid for the data but were duped.

"We're investigating this allegation and have submitted a takedown request to the forum that's advertising the claimed data," Facebook stated in a statement.

Although there were rumors of problems, Twitter was operational enough for the site – and CEO Jack Dorsey – to poke fun at Facebook. As people flocked to Twitter, the site posted, "hey practically everyone."

And Dorsey asked "how much?" in response to the Facebook.com domain auction ad.

Instagram had to take to Twitter to inform its users that it was aware of the issue and was trying to resolve it. "Instagram and friends are having a little bit of a hard time right now, and you may be experiencing problems utilizing them," the website tweeted. "Please bear with us; we're working on it!"