The outage was the second blow to the social media giant in as many days after a whistleblower on Sunday accused the company of repeatedly prioritising profit over clamping down on hate speech and misinformation.
"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 per cent".
Shares of Facebook, which has nearly two billion daily active users, fell 4.9 per cent on Monday, their biggest daily drop since last November, amid a broader selloff in technology stocks. Shares rose about half a per cent in after-hours trade following resumption of service.
Security experts said the disruption could be the result of an internal mistake, though sabotage by an insider would be theoretically possible.
"Facebook basically locked its keys in its car," tweeted Jonathan Zittrain, director of Harvard's Berkman Klein Center for Internet & Society.
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 by the outage.
Facebook later blamed the outage on configuration changes it made to routers that coordinate network traffic between its data centres.
"This disruption to network traffic had a cascading effect on the way our data centers communicate, bringing our services to a halt," Facebook vice president of infrastructure Santosh Janardhan said in a post.
In Australia, outages began being reported by users of Facebook, Facebook Messenger, Instagram, WhatsApp from about 3am AEDT local time on Tuesday.
Thousands of users reported issues accessing Facebook websites and aaps, according to Downdetector's Australian site.
Facebook, which is the second-largest digital advertising platform in the world, was losing about $747,880 AUD in US ad revenue per hour during the outage, according to estimates from ad measurement firm Standard Media Index.
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 Wall Street Journal investigation and a Senate hearing on Instagram's harm to teen girls.
Ms Haugen is due to urge the US Congress on Tuesday local time to regulate the company.