Facebook and its properties WhatsApp, Instagram, and others went offline for about five hours yesterday.
Many small businesses throughout the world depend on these services to reach customers. They got a rude shock.
Why did this outage happen?
There was nothing wrong with Facebook's computers; they were running fine. The problem was their Internet connectivity.
Kingdoms---Autonomous Systems
The Internet has no government; it is made up of hundreds of thousands of sovereign kingdoms. One of these is the Kingdom of Facebook. (Actually, Facebook runs a handful of kingdoms, but they're managed as one single empire.)
Kingdoms like Facebook advertise Internet services like whatsapp.com running on its own computers. To access any Internet services, your computer must be in one of these kingdoms.
If you're using data, then your mobile phone is in your phone company's kingdom. If you're on WiFi, then it's in your Internet Service Provider's kingdom.
In either case, when you type a WhatsApp message, your phone sends data packets addressed to "whatsapp.com" to your phone's own kingdom. Your phone company or ISP knows that the "whatsapp.com" service is maintained by the Facebook kingdom, so it forwards these data packets to Facebook.