DNS issues are the cause for so many outages by a myriad of prestigious web shops. It's incredible how often overlooked something so pivotal to your infrastructure goes without being properly monitored. I have seen entire businesses suffer hours of downtime because their monitoring systems would query an authoritative name server from RFC1918 IP space - but not the same from an externally visible address. An NS not responding to local queries can be just as detrimental internally as they are externally because of how web services are architected, obviously. In their defense, Facebook rarely incurs outages of this capacity, and I can bet it won't happen again and someone is getting seriously reamed. They can and will happen to the best of them, no matter how many MIT PhDs you have designing your systems.