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

The company in a late Monday (US time) blog post did not specify who executed the configuration change and whether it was planned.

Several Facebook employees who declined to be named had told Reuters earlier that they believed that the outage was caused by an internal mistake in how Internet traffic is routed to its systems.

The failures of internal communication tools and other resources that depend on that same network in order to work, compounded the error, the employees said. Security experts have said an inadvertent mistake or sabotage by an insider was both plausible.

“We want to make clear at this time we believe the root cause of this outage was a faulty configuration change,” Facebook said in the blog.

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 prioritizing profit over clamping down on hate speech and misinformation.

As the world flocked to competing apps such as Twitter and TikTok, shares of Facebook fell 4.9 percent, their biggest daily drop since last November, amid a broader selloff in technology stocks on Monday.

Meanwhile, Russian social networks reported a spike in activity during Monday’s global Facebook outage, which Moscow officials said showed that Russia was right to develop its own sovereign Internet platforms and social networks, Reuters reported from Moscow.

Russia has sought for years to assert greater sovereignty over its Internet segment, putting pressure on foreign tech firms to delete content and store data in Russia. It has also improved its ability to block platforms that break its rules.

Maria Zakharova, Russia’s Foreign Ministry spokesperson, said during the near six-hour outage of Facebook services on Monday evening this “answers the question of whether we need our own social networks and Internet platforms”.