It's done through BGP.

If you're IT savvy, you can read this from Cisco and see how it's done-

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/irg-xe-16-book/removing-private-as-numbers-from-the-as-path-in-bgp.html

https://archive.is/jG4ke

Edit: Something similar to this had to have happened as even Facebook's internal systems are down. Techs would have had to bash the doors in to get to the servers as even the entry badges aren't working.

It's done through BGP.

If you're IT savvy, you can read this from Cisco and see how it's done-

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/irg-xe-16-book/removing-private-as-numbers-from-the-as-path-in-bgp.html

https://archive.is/jG4ke