I hate Facebook
I see that it took Facebook literally one day to update their iPhone app to fit in with the aesthetic of iOS7:
Meanwhile, on Android, it's been nearly two years since the introduction of ICS, and we're still waiting for a Holo-themed Facebook app.
Now, Apple fans will no doubt say that proves the superiority of the Apple-style centralised update model. And to some extent, this is true - it certainly does sharpen incentives for developers when the majority of users can update to the latest version of the OS right away.
But that doesn't fully explain the situation. Pretty much every other "big-name" app on Android has been redesigned to fit in with the new Android aesthetic. Facebook (and I guess Instagram, which is owned by Facebook) is just about the only big holdout left. I think the correct answer is also the simplest: Facebook is a lazy, arrogant developer. Period.