So Facebook is yet another of those companies where being hired depends on CS/math trivia questions? Apparently a lot of people think it's easier to design a good API than look up Newton's method on Wikipedia.
Not to diminish the awesome work this guy has done.
That question would have made more sense for an engine in C, building your own fixed-point sqrt() for example.
Questions about trees, graphs etc. would seem more appropriate for game development (and probably for Facebook in general).
But that's just me. Sqrt might even be a good question if he's a recent graduate and they want to see if he still can think back a couple of years...