Facebook said it fixed networking issues that made its flagship app — as well as Instagram and WhatsApp — inoperable Monday in an outage lasting more than six hours.

Users reported a spike in problems accessing Facebook’s family of services starting around 11:35 a.m. ET. The issues preventing users from posting content or refreshing their feeds, while the websites of Facebook and Instagram displayed error messages to visitors. By 6 p.m. ET, some of the social giant’s services had become available again.

The outage was the longest downtime for Facebook since March 2019, when the service was down for about 24 hours across multiple apps.

Facebook apologized in a statement posted to Twitter.

“To the huge community of people and businesses around the world who depend on us: we’re sorry,” the company tweeted from @Facebook at 6:33 p.m. ET. “We’ve been working hard to restore access to our apps and services and are happy to report they are coming back online now. Thank you for bearing with us.”

Experts identified Facebook’s problems Monday as related to a configuration change in the company’s domain name system (DNS) entries, which effectively made Facebook, Instagram, WhatsApp and Messenger temporarily invisible to internet users worldwide.

Facebook CTO Mike Schroepfer apologized for the issues in a tweet Monday afternoon, attributing the downtime to unspecified “networking issues.”

In a blog post Monday evening, Facebook VP of infrastructure Santosh Janardhan said the company’s engineering teams identified the root cause of the as “a faulty configuration change” on the backbone routers that coordinate network traffic between its data centers. “This disruption to network traffic had a cascading effect on the way our data centers communicate, bringing our services to a halt,” Janardhan wrote, adding, “We also have no evidence that user data was compromised as a result of this downtime.”

The blackout of Facebook and Instagram drove up usage of Twitter, which acknowledged that the surge stressed its own servers. “Sometimes more people than usual use Twitter. We prepare for these moments, but today things didn’t go exactly as planned,” Twitter Support said in a tweet at 4:55 p.m. ET. “Some of you may have had an issue seeing replies and DMs as a result. This has been fixed. Sorry about that!”

The DNS error also prevented some Facebook employees from being able to access email and using company-issued mobile phones, and knocked out its internal communications system, Workplace, the New York Times reported. That forced staffers to resort to non-Facebook email and communications tools.

In response to a tweet that Instagram should “stay offline forever” by photographer Noah Kalina, Adam Mosseri — the head of Instagram — replied, “Them fighting words… but it does feel like a snow day.”

The extended outages of Facebook’s apps — which in total average 2.76 billion daily active users worldwide in June 2021 — came after a damaging exposé last month by the Wall Street Journal. The series, dubbed the “Facebook Files,” found that company execs have routinely been made aware of multiple problems and have not taken action to fix them. On Sunday, ex-Facebook exec Frances Haugen revealed herself to be the whistleblower who provided the documents to the Journal and the SEC.