Facebook (FB  ) has apologized and somewhat explained the reason behind the mass outage that left users unable to access Facebook, Instagram, WhatsApp and Messenger for several hours on Monday.

"To all the people and businesses around the world who depend on us, we are sorry for the inconvenience caused by today's outage across our platforms," said Santosh Janardhan, vice president of infrastructure at Facebook, in a blogpost late Monday. "We've been working as hard as we can to restore access, and out systems are now back up and running."

Janardhan explained that the outage was caused by "configuration changes on the backbone routers that coordinate network traffic between our data centers caused issues that interrupted this communication," but did not clarify exactly what the changes were.

"This disruption to network traffic had a cascading effect on the way our data centers communicate, bring our services to a halt," Janardhan said. "We want to make clear that there was no malicious activity behind this outage--its root cause was a faulty configuration change on our end."

Facebook services, including Instagram and WhatsApp, had stopped working shortly before noon ET on Monday, with both websites and apps not allowing users to refresh and Facebook's services responding with server errors.

The outage marked the longest period of downtime for Facebook since 2008, when a bug took the social media platform offline after about a day. Facebook also had a similar outage back in 2019 when a server configuration change made the website go dark for about an hour.

Janardhan said that Monday's disruption to network traffic has not compromised user data.

"We apologize to all those affected, and we're working to understand more about what happened today so we can continue to make our infrastructure more resilient," Janardhan added.

Facebook's outage came at a bad time for the social media giant as the whistleblower behind the leaked Wall Street Journal information, Frances Haugen, is set to testify before Congress on Tuesday about her experiences working for the company. The internal documents leaked to WSJ revealed that Facebook executives knew that Instagram negatively impacted younger users and that it's algorithm heavily enables the spread of misinformation.

Facebook shares closed lower alongside the slumping market on Monday, falling nearly 5% by closing bell.