Facebook has stacks of thousands of spare nodes ready at any moment to replace a failed node. All essential data will be replicated across many different boxes so if a box fails you just replace it with a fresh node and replicate the data there.
This is much different to the consumer usecase where computers are pets and not cattle. A failed filesystem the night before you need to turn in your thesis may have a much larger impact on your life.
Another thing to consider is that Facebook runs btrfs on enterprise hardware (including SSDs with battery backups) which is going to be much more reliable than some chromebook which lives in the bottom of your backpack that you bring on transit every day.
Finally, I will say that the copy on write features of btrfs can result is some wildly different behaviour based upon how you use it. You can get into some very bad pathological cases with write amplification, and if you run btrfs on top of LUKS it can nearly be impossible to figure out why your disk is being pegged despite very little throughput at the VFS layer.
