It’s rare that such a widely established network of sites, such as Facebook and it’s associated platforms, can experience a widespread outage that lasts for hours.
While no official reason has been given, there are some who are speculating that it was a domain name system (DNS) issue.
For those who like tech details, Wired.com gives a good idea of what may have happened.
There’s also this Twitter thread. I don’t know how authentic the original source is, but the consensus is that it does make sense.
As of this writing, Facebook has reported that their sytems are coming back online.
Source: BBC News