People thought the same about YouTube, but that turned out to be a brilliant (and very profitable) move.
Then again, they also thought the same about AOL/TimeWarner, and they were absolutely right then.
It's usually pretty futile to try to predict the future. I can see Zuck's reasoning with the Whatsapp purchase, though, and I don't think he's insane. He and Larry probably understand the structure of the tech industry a lot better than most people.