Social media giants Facebook, WhatsApp, and Instagram went down for 6 hours on Monday night. This was one of the biggest outages in Internet history with more than 10 million people affected at once.

Facebook has explained the cause of this massive outage in a blog post shared this morning. The disruption that brought down Facebook, Instagram, and WhatsApp was not the result of a hack or breach, but a change of configuration to Facebook’s routers.

The routers that coordinate network traffic between data centers at Facebook interrupted communication. The blog post does not explain the disruption in detail, but it appears that Facebook’s machines were not able to communicate with each other properly for some unspecified reason.

This disruption to network traffic had a cascading effect on the way our data centers communicate, bringing our services to a halt.

Facebook explains that the outage also affected many of their internal tools and systems used in day-to-day operations, which is why resolving the problem became more complicated.

CEO Mark Zuckerburg shared an apology post on Facebook on Monday night which said:

Sorry for the disruption today — I know how much you rely on our services to stay connected with the people you care about.

Facebook, Instagram, and WhatsApp are now back online once again and have been working properly for almost 8 hours.