It seems like an easy redundancy split, but imagine driving two cars down the freeway at the same time, because you got a flat tire in one, the other day.
In order to actually be redundant you need to have two sets of infrastructure to serve, and then if the internal one goes down, the external one's basically useless when the internal resolution's down anyway. Capacity planning (because you're inside Facebook and can't pretend that all data-centers ever-where are connected via an infinitely fast network) becomes twice as much work. How you do updates for a couple thousand teams isn't trivial in the first place, now you have to cordon them off appropriately?
I don't know what Facebook's DNS serving infrastructure looks like internally, but it's definitely more complicated than installing `unbound` on a couple of left-over servers.
