Facebook finally gave a reason for the six-hour outage on Monday

Facebook stated in a blog post late Monday that the six-hour outage, which also affected Instagram, Messenger, Whatsapp, and OculusVR, was caused by a router configuration change, not a hack or an attempt to steal user data.
The explanation is sparse, but it appears that Facebook’s machines are unable to interact with one another — Facebook claims that “this disruption in network traffic had a cascade effect on the way our data centers communicate. to bring our services to a halt.”

Here's what people did during Facebook, Instagram outage
On Monday night, Facebook CEO Mark Zuckerberg apologized and stated that the platforms would be restored.

“Sorry to bother you today ̵ I know how much you rely on our services to keep in touch with the people you care about.”

The outage began about 11:40 a.m. ET on Monday and caused the company significant troubles.
It was Facebook’s worst outage since January of this year, when the site was down for over 24 hours.
Employees were unable to communicate with one another on the company’s message boards, according to some.
They communicated using Outlook e-mail addresses provided by their employers.

Facebook, WhatsApp and Instagram go down in major, worldwide outage | WUNC

The issues appeared to stem from a normal BGP update that went wrong, erasing the DNS routing information that Facebook requires for other networks to find its sites.

The demise of Facebook occurred just one day before whistleblower Frances Haugen was scheduled to testify before Congress about her time there.
The Wall Street Journal was referred to reporters by Haugen, a former Facebook product manager who worked on the company’s Civic Integrity department.
Facebook “pays its profits with our security,” she claimed on 60 Minutes on Sunday.