Cloudflare, a CDN provider, wrote up an excellent article on how Facebook disappeared from the Internet on 2021 October 4. It refers to Facebook’s own update about the October 4 outage.

The Cloudflare article is really informative, because it explains the engineering behind the problem (Facebook issued some DNS changes that effectively removed it from being accessible by name), but here’s a summary from Facebook’s article that is probably better for quick consumption:

Our engineering teams have learned that configuration changes on the backbone routers that coordinate network traffic between our data centers caused issues that interrupted this communication. This disruption to network traffic had a cascading effect on the way our data centers communicate, bringing our services to a halt.