The apps went dark early about 5.30am today (NZ time) in what website monitoring group Downdetector said was the largest such failure it had ever seen. There were 10.6 million problem reports globally.
About 11.30am (NZ time) some Facebook users began to regain access.
Facebook apologised but did not immediately explain what caused the failure.
As the world flocked to competing apps such as Twitter and TikTok, shares of Facebook fell 4.9%, their biggest daily drop since November last year.
"To every small and large business, family, and individual who depends on us, I'm sorry," Facebook Chief Technology Officer Mike Schroepfer tweeted, adding that it "may take some time to get to 100%."
Several Facebook employees who declined to be named said that they believed that the outage was caused by an internal mistake in how internet traffic is routed to its systems. The failures of internal communication tools and other resources that depend on that same network in order to work compounded the error, the employees said.
Security experts said the disruption could be the result of an internal mistake, though sabotage by an insider were both plausible.
"Facebook basically locked its keys in its car," tweeted Jonathan Zittrain, director of Harvard's Berkman Klein Center for Internet & Society.
Twitter on Monday reported higher-than-normal usage, which led to some issues in people accessing posts and direct messages.
In one of the day's most popular tweets, video streaming company Netflix shared a meme from its new hit show Squid Game captioned "When Instagram & Facebook are down," that showed a person labelled "Twitter" holding up a character on the verge of falling labelled "everyone."
Inside a Facebook group for ad buyers, one member wisecracked after service returned that "lots of people searched today 'how to run google ads for clients.'"
Facebook, which is the world's largest seller of online ads after Google, was losing about $US545,000 ($NZ784,940) in US ad revenue per hour during the outage, according to estimates from ad measurement firm Standard Media Index.
Past downtime at internet companies has had little long-term affect on their revenue growth, however.
Soon after the outage started, Facebook acknowledged users were having trouble accessing its apps but did not provide any specifics about the nature of the problem or say how many users were affected.
The error message on Facebook's webpage suggested an error in the Domain Name System (DNS), which allows web addresses to take users to their destinations. A similar outage at cloud company Akamai Technologies Inc took down multiple websites in July.
On Sunday, Frances Haugen, who worked as a product manager on the civic misinformation team at Facebook, revealed that she was the whistleblower who provided documents underpinning a recent Wall Street Journal investigation and a US Senate hearing last week on Instagram's harm to teen girls.
Haugen was due to urge the same Senate subcommittee on Tuesday to regulate the company, which she plans to liken to tobacco companies that for decades denied that smoking damaged health, according to prepared testimony seen by Reuters.
