I can't speak beyond 2011, but in the early part of the last decade you're giving Facebook far too much credit.

Facebook went down for most of a day in ~2009 because a new hire mistakenly removed the memcached server config in sitevars.

Facebook went down for several hours in ~2010 because someone configured a cyclic dependency in GateKeeper.

Circa 2011, Facebook's deployment process was very good, but also very very far from infallible.