Update: Oops, I meant TTL.. not ARP :D
The reason being is most users connect to Facebook or Instagram DAILY... which means their TTL cache would still be applied...
This happened either several days ago and users are just now starting to be affected as TTL cache expires... or something more deep. Actual routing problems.
This could be the start of the take down and the 10 days.. I hope all of us are prepared if so.
This is only due to your own cache locally.
do
ipconfig /flushdns
then try
nslookup facebook.com