I wrote this as a pure joke, but now that I learned that SERVFAIL is not cached on browsers, clients, intermediate DNS servers [0] etc. I am curiously wondering what will be going on. It is not only FB apps, it is basically every website request (that uses FB JS for ads, tracking, etc.) that triggers a DNS request, which will be forwarded 1:1 from the ISP's DNS to the null-routed FB Subnet. This should put orders of magnitude more load on resolving DNS servers than usual.
[0]: https://serverfault.com/questions/479367/how-long-a-dns-time...