Facebook on Monday said that it was a "faulty configuration change" which caused nearly a six-hour global outage preventing users from accessing its social media and messaging services such as WhatsApp, Instagram and Messenger.
However, Facebook did not say who made the configuration modification or whether it was planned.
"Our engineering teams have learned that configuration changes on the backbone routers that coordinate network traffic between our data centres caused issues that interrupted this communication. This disruption to network traffic had a cascading effect on the way our data centres communicate, bringing our services to a halt." Facebook said.
"We want to make clear at this time we believe the root cause of this outage was a faulty configuration change," Facebook said in a blog post.
Several Facebook employees who wanted to remain anonymous told news agency Reuters that they believed that the outage was caused by an internal mistake in how internet traffic is routed to its systems.
According to web monitoring group Downdetector, this was the longest ever Facebook outage.
As users flocked to competing apps like Twitter and TikTok, Facebook's stocks dropped by 4.9 per cent on Monday, its worst daily drop since November, as part of a broader selloff in technology firms.
"To every small and large business, family, and an 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%."
Mark Zuckerberg, the CEO of Facebook, Whatsapp, and Instagram, has apologised for the outage and promised that the services will be back online on Tuesday.
"Facebook, Instagram, WhatsApp and Messenger are coming back online now," Zuckerberg said in a Facebook post.
"Sorry for the disruption today -- I know how much you rely on our services to stay connected with the people you care about," he said.