Perhaps, but once you're as big as FB, you should be able to guess that your growth rate will inevitably slow. No company that big keeps growing at the same rate, or they'd take over the entire economy in short order. AT&T, GE, IBM, Dell, Microsoft, all had their growth rate slow as they got big. If FB was hiring on the assumption that it wouldn't happen to them, they were foolish to do so.

It's also a way to take advantage of high-priced stock while they can. Of course that doesn't mean they should hire recklessly and pay through the nose for people to do VR research or whatever that won't pay off this decade.