I really, really, really wish people would STOP with the whole "it works for $SilconValleyCorp so it must work for me" or "$SiliconValleyCorp does it, so I must".
It only leads to disappointment in the case of the former and wholly un-necessary over-engineering in the case of the latter.
(a) You do not know *how* or *where* Facebook use BTRFS
(b) Even in the unlikely event they use it "everywhere", they have far more redundancy on every layer than you will ever have. So they don't care if a random BTRFS instance borks itself.
(c) Facebook probably employ the guy who invented BTRFS and an army of kernel developers on top of that .... how much in-house support do you have for BTRFS ?
As far as I am concerned, the fact that they STILL have not fixed RAID5 in BTRFS says everythng you need to know.