What caused the Facebook outage? Here's what we know.

Officials with Facebook said on Monday that a "faulty configuration change" was to blame for its worldwide outage that lasted for several hours. Instagram, What'sApp and Facebook were not accessible to anyone for more than 6 hours.

The company said no user data had been compromised, NBC News reports.

Experts say it appeared to be a problem with the Domain Name System, the "phone book" of the internet, which computers use to look up individual websites. In fact, "Facebook.com" was no longer listed on the DNS.

By not having a domain name, people on the internet weren't able to find Facebook, Instagram or What'sApp when they tried connecting to the sites.

According experts, it appears that when the configuration was updated, there was a mistake in the code. Cyber threat analysts told Wired that Facebook withdrew its Border Gateway Protocol route that contains the IP addresses of its DNS nameservers. The BGP is how the internet accesses its DNS names, or how it connects to a webpage.

Facebook has not said what exactly caused the outage.

Experts say that people trying to refresh Facebook on their phones or computers, over and over again, likely prolonged the outage.

Internal services at Facebook, What'sApp and Instagram were also down, like employee's keycards, the New York Times reported.

A Facebook employee told NBC News that there was no reason to suspect foul play was involved.

In only a couple hours, CEO of Facebook Mark Zuckerberg lost billions of dollars in net worth as Facebook's stock fell. The web crash came hours after a ex-Facebook employee was interviewed on 60 minutes about the social media apps harming teenagers and perpetuating misinformation.

The whistleblower was called to testify before Congress on Tuesday.