Surely Facebook don't update routing systems between data centres (IIRC the situation) when they don't have people present to fix things if they go wrong? Or have an out-of-band connection (satellite, or dial-up (?), or some other alternate routing?).

I must be misunderstanding this situation here.

[Aside: I recall updating wi-fi settings on my laptop and first checking I had direct Ethernet connection working ... and that when I didn't have anything important to do (could have done a reinstall with little loss). Is that a reasonable analogy?]