[homebrewer]

> You do not know how or where Facebook use BTRFS

(S)he does, their employees explained it many times. They're very public about it.

> So they don't care if a random BTRFS instance borks itself.

They do, according to Christ Mason (IIRC) they investigate every instance of btrfs corruption, regardless of how unimportant the machine and data were. They're not any more frequent than with any other filesystem.

> Facebook probably employ the guy who invented BTRFS and an army of kernel developers on top of that

Not an "army" (only a few developers), but you're correct here.

> they STILL have not fixed RAID5 in BTRFS

Why would they? It's a niche technology that's only interesting to a few home users. I am a home user and have no use for it (or any of the alternatives like raidz).