When you're as large as Google, these kinds of talent losses are natural. Take, for example, Marissa Mayer, disregarding whatever people on HN might think of her (she almost certainly represents one of those 'Key Employee' numbers). She clearly has ambition, and she was clearly not going anywhere at Google, so the only thing to do was to leave.

I also suspect that the challenges Twitter faces are more business-oriented - they probably attract a different type of talent.

TL;DR Google is frickin' huge and just plain different than Twitter. It makes no sense to compare the two this way.