As of about 15 minutes ago, Facebook and its associated companies, Instagram, Whatsapp, etc are current offline, mainly due to a DNS resolution issue. This may be a deliberate change to protect the servers, or it could be a failure of the DNS resolvers. No one really knows at this point.
We can verify this by doing DNS resolution checks from the command line, and we get
; <<>> DiG 9.10.6 <<>> -t A www.facebook.com @22.214.171.124 +all
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 436
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;www.facebook.com. IN A
;; Query time: 23 msec
;; SERVER: 126.96.36.199#53(188.8.131.52)
;; WHEN: Mon Oct 04 18:03:09 BST 2021
;; MSG SIZE rcvd: 45
The quick response means that Google (184.108.40.206) has already cached the failure and is returning nothing for www.facebook.com. There's like to be a short delay once its back up before resolvers expire this and retry, depending on the zone configuration.