Cheers, I'd edited my post.

Thought the common wisdom nowadays was to use nameservers on different TLDs and sub-labels for the best resilience.

/added, they seem to have glue records so I'd assume it's the nameservers themselves having issues.

$ dig NS @g.gtld-servers.net. a.ns.facebook.com

;; AUTHORITY SECTION:

facebook.com. 172800 IN NS a.ns.facebook.com.

facebook.com. 172800 IN NS b.ns.facebook.com.

facebook.com. 172800 IN NS

c.ns.facebook.com.

facebook.com. 172800 IN NS d.ns.facebook.com.

;; ADDITIONAL SECTION:

a.ns.facebook.com. 172800 IN A 129.134.30.12

a.ns.facebook.com. 172800 IN AAAA 2a03:2880:f0fc:c:face:b00c:0:35

b.ns.facebook.com. 172800 IN A 129.134.31.12

b.ns.facebook.com. 172800 IN AAAA 2a03:2880:f0fd:c:face:b00c:0:35

c.ns.facebook.com. 172800 IN A 185.89.218.12

c.ns.facebook.com. 172800 IN AAAA 2a03:2880:f1fc:c:face:b00c:0:35

d.ns.facebook.com. 172800 IN A 185.89.219.12

d.ns.facebook.com. 172800 IN AAAA 2a03:2880:f1fd:c:face:b00c:0:35