It might be about the size of the screen and whether or not you’re standing up.
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.