Social media giant Facebook Inc has blamed a “faulty configuration change” for the nearly six-hour outage on Monday that prevented the company’s 3.5 billion users from accessing its services, including that of WhatsApp, Instagram and Messenger.

“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.

Soon after the outage happened, Facebook acknowledged the trouble users were facing accessing its apps but did not provide any specifics about the nature of the problem.

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.

Several Facebook employees who declined to be named have told Reuters they believed the outage was caused by an internal mistake in how internet traffic is routed to its systems. They said the failures of internal communication tools and other resources that depend on that same network in order to work compounded the error. Security experts have said an inadvertent mistake or sabotage by an insider could be possible reasons.

The Facebook outage is the largest-ever tracked by web monitoring group Downdetector. 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.

The shares of Facebook fell 4.9 per cent, their biggest daily drop since last November, amid a broader selloff in technology stocks on Monday. “To every small and large business, family, and individual who depends on us, I’m sorry,” Facebook chief technology officer Mike Schroepfer wrote on Twitter, adding that it “may take some time to get to 100 per cent”.

(With inputs from Reuters)