Facebook and all of its apps go down simultaneously.

Facebook and its family of apps, including Instagram and WhatsApp, went down at the same time on Monday, taking out a vital communications platform used by billions and adding heat to a company already under intense scrutiny.

Facebook’s apps — which include Facebook, Instagram, WhatsApp, Facebook Messenger and Oculus — began displaying error messages around 11:40 a.m. Eastern time, users reported. Within five minutes, Facebook had disappeared from the internet. Hours later, the sites were still not functioning, according to Downdetector, which monitors web traffic and site activity.

The impact was immediate and global. Facebook has built itself into a linchpin platform with messaging, livestreaming, virtual reality and many other digital services. In some countries, like Myanmar and India, Facebook is synonymous with the internet. More than 3.5 billion people around the world use Facebook, Instagram, Messenger and WhatsApp to communicate with friends and family, distribute political messaging, and expand their businesses through advertising and outreach.

The fallout also cascaded as Facebook is used to sign in to many other apps and services. That led to unexpected domino effects such as people not being able to log into shopping websites or sign into their smart TVs, thermostats and other internet-connected devices.

Technology outages are not uncommon, but to have so many apps go dark from the world’s largest social media company at the same time was highly unusual. Facebook’s last significant outage was in 2019, when a technical error affected its sites for 24 hours, in a reminder that even the most powerful internet companies can be crippled by a snafu.

This time, the cause of the outage remained unclear. It was unlikely that a cyberattack was the culprit because a hack generally does not affect so many apps at once, said two members of Facebook’s security team, who spoke on the condition of anonymity. Security experts said the problem most likely stemmed instead from a misconfiguration of Facebook’s server computers, which were not letting people connect to its sites like Instagram and WhatsApp.

Andy Stone, a Facebook spokesman, posted on Twitter, “We’re aware that some people are having trouble accessing our apps and products. We’re working to get things back to normal as quickly as possible, and we apologize for any inconvenience.”

The outage adds to Facebook’s mounting difficulties. For weeks, the company has been under fire from a whistle-blower, Frances Haugen, a former Facebook product manager who amassed thousands of pages of internal research. She has since distributed the cache to the news media, lawmakers and regulators, revealing that Facebook knew of many harms that its services were causing, including that Instagram made teenage girls feel worse about themselves.

The revelations have prompted an outcry among regulators, lawmakers and the public. Ms. Haugen, who revealed her identity on Sunday online and on “60 Minutes,” is scheduled to testify on Tuesday in Congress about Facebook’s impact on young users.

When the outage began on Monday morning, Facebook and Instagram users quickly turned to Twitter to lament and poke fun at their inability to use the apps. The hashtag #facebookdown also started trending. Memes about the incident proliferated.