- the DNS services internally have issues (most likely, as this could explain the snowball effect)
- it could be also a core storage issue and all their VMs are relying on it and so they don't want to block third-party websites and think it will last for a long time, so they prefer to answer nothing for now in the DNS (so it will fail instantly to the client, and drain the application/database servers so they can reboot with less load)
I was on a video call during the incident. The service was working but with super-low bandwidth for 30 minutes, then I got disconnected and every FB property went down suddenly. Seems more suggestive of someone pulling the plug than a DNS issue, although it could also be both.
At first it was working but they couldn't serve responses: https://i.imgur.com/UaCtOiX.png
Notice the "2020"