It’s in the last two categories that these other devices, things that don’t involve sitting down, are superior, not just a mobile substitute. The social graph is a very low bandwidth, peripheral attention interaction, perfect for this audience and this medium. And the last category–tell me where I am, where to eat, who’s near me, what’s the weather, get me a cab right now–is all about me and now and here.
I don’t believe this is a winner take all situation, any more than one bestselling book makes all other books obsolete. I think different pillars work for different devices, and there will continue to be winners in all of them.