I might disagree that it's a poor use of Mark's time. The only product that Facebook has is software and a large portion, I'm guessing, comprises of internal APIs. So, what better way to test their quality than by writing some code against them? I don't know if this is what he's doing or not, but it seems like a logical guess.

I would expect the captain of a ship to make sure the engine is running correctly, even if that means a personal inspection.

Plus, from what I've read, a lot of internal discussions at Facebook are settled by writing demo code/applications showing a concrete implementation of an idea.