Sure, but did you notice how they use Facebook? I've watched (and asked) several of my non-techy friends and there reaction was always the same: "Yeah, I just update my feed, look for updates of my friends and that's it."
I don't see how Facebook wants to make money of such an usage pattern, so at the moment it doesn't help them very much that people have smart phones to "do facebook".
Quite the opposite, in my experience. I can read my feed on my 'phone, with a better UI than the website (turns out native app beats webapp, even when it's tiny screen vs. desktop), so why would I ever bother going to the website?
Most of my FB friends seem to use the mobile app mainly for uploading photos, if my feed is to be believed. From that POV the Instagram acquisition makes sense. How they're going to make money from it I'm not sure, but location-based ads and third-party apps come to mind. Though I still have a hard time imagining the developer appeal for their hypothetical but inevitable mobile app platform.
I don't see how Facebook wants to make money of such an usage pattern, so at the moment it doesn't help them very much that people have smart phones to "do facebook".