Name servers don't work that way. It doesn't matter where their primary name servers are hosted, because their records propaganda to all global name servers and you use one close to you for DNS lookups.

Facebook may or may not have name servers hosted outside the US, the only 2 IP blocks mentioned in this Cloudflare blog post are both US ones.

Regardless, they withdrew the routes for the IP blocks of the data centers where all their DNS servers are hosted, so there was no longer any primary DNS server with records for any of their domains on the internet. Once the TTL expired on the records, the name servers closest to you would have tried to query the domains and would not be able to reach any data center where a primary DNS server was hosted.