Facebook has an ASN. Think of this as a big network of computers, for which Facebook defines the routing policy (how traffic has to jump from router to router to reach the destination machine). But their network also has to talk to outside networks, called peers. For updating routing information with the peers, they use a standard protocol, BGP. It's an automatic system which allows Facebook admins to propagate routing information to their peers. So when they change routing on their network, BGP tells their neighbors about it.

What happened was that shortly before the blackout, Facebook sent a lot of BGP "withdrawal" messages which basically told the peers that the computers in their ASN are not there anymore. The peers complied and deleted the routing information.

The result is now that no one on the internet knows how to route to any of Facebooks addresses anymore. This includes ALL of Facebooks network. Internal communication systems, Keycard access to server rooms, ability to remotely change routing configuration: All gone.

They now have to break into their datacenters and plug cables into routers.