Not saying this is anything, but maybe someone else on here will see this and have a lightbulb moment...
All 3 DNS records for Facebook, Whatsapp and Instagram were updated on Sep 22.
https://whois.domaintools.com/facebook.com
https://whois.domaintools.com/whatsapp.com
https://whois.domaintools.com/instagram.com
I also noticed the same date on the other domains:
https://whois.domaintools.com/whatsapp.net
https://whois.domaintools.com/facebook.net
I am sure there are others. This could be just a simple Zuckerberg and Co. had something they changed/updated on that date for the entire FB empire, but I find a massive outage a few days (12) later to be really strange.
I do not have a whois premium account so I am unable to dig much further than this. Anything interesting happen on Sep 22?
It entirely depends on the TTL value for the record.. some put very high values (decreases likelihood of DNS based attacks).. generally bigger sites will have a lengthly value here (in seconds... thousands of seconds generally).