Facebook is still down, at least six hours after the outage began.
The website and app began experiencing issues early on Thursday morning, dragging WhatsApp and Instagram down with it.
All three services are owned by the same company.
BBC Silicon Valley correspondent Dave Lee said the outage is unmatched in its length and breadth.
"The length of time, plus number of users affected, makes this comfortably the most severe outage in Facebook's history," he said on Twitter.
"Last period of down time longer than this was in 2008, when Facebook had a mere 50m users."
But there's no indication about what caused the outage and Facebook itself is tight-lipped about the whole affair.
"We're aware that some people are currently having trouble accessing the Facebook family of apps. We're working to resolve the issue as soon as possible," it said on Twitter.
"We're focused on working to resolve the issue as soon as possible, but can confirm that the issue is not related to a DDoS attack."
A similar message was posted to Instagram's Twitter just before 12pm on Thursday.
"We're aware of an issue impacting people's access to Instagram right now. We know this is frustrating, and our team is hard at work to resolve this ASAP," it said.
"Well work harder," one user replied.