A simple explanation of Facebook's massive outage and why it took so long to fix

• Facebook said changes to its "backbone routers" caused Monday's widespread outage.
• Facebook said it had "no evidence that user data was compromised" as a result of the outage.
• The global outage impacted all of Facebook's apps and even internal communications.

Facebook published a blog post late Monday attributing its hours-long global outage to a "faulty configuration change" to its "backbone routers."

In the most basic terms, this means a technical update ground traffic to a halt on the highway most important to the company's operations.

Specifically, traffic was interrupted between Facebook's data centers, which are the physical buildings housing the company's servers and IT equipment.

These data centers power everything Facebook does - from its apps like Instagram and WhatsApp to the company's internal communications.

So the traffic interruption didn't just kick the company's apps offline. It also cut off Facebook employees from sending messages or fixes internally. And it stopped them from physically accessing the impacted servers when their company badges - which can unlock doors at Facebook facilities - failed.

This made it especially tricky for Facebook to solve the problem. The company ultimately restored service by sending a team to reset its servers at a data center in Santa Clara, California, the New York Times reported, citing three people with knowledge of the matter.

Facebook said it had "no evidence that user data was compromised as a result of this downtime."

The company's blog post confirmed that Facebook's services are "back online" and apologized for the inconvenience.

The outage impacted Facebook and all of its most widely used apps, including Instagram, WhatsApp, and Messenger.

Some services appeared to be back online for some users about 6 p.m. ET.

In the meantime, the outage wreaked havoc for influencers and businesses who rely on the platforms, users from across the world, and shareholders including CEO Mark Zuckerberg.

Zuckerberg put a statement on his own Facebook page apologising for the outage on Monday just before 7 p.m. ET.

"Sorry for the disruption today - I know how much you rely on our services to stay connected with the people you care about," Zuckerberg said.