I've had this thought cross my mind, too... some time ago, actually. My smartphone, a Windows Phone, is a natural social network browser; a mature response to "how to make social networking easy, practical, and physically portable".
I would roughly guestimate that a good 50% of the OS, fresh out of the box, is dedicated to it (particularly the part known as the "People Hub"). Interestingly, the People Hub isn't a Facebook app. It's exactly what it sounds like... a contacts hub, a leads hub, management hub, a communications hub, a social hub... your people hub. Twitter, Facebook, LinkedIn, multiple Exchange domains, even Windows Live.
Some time ago I actually deleted the Facebook app from my phone, because I couldn't think of a good reason to let it take up space on my phone. That was when it hit me. Aside from the basic description in settings here & there, the Facebook brand did not really exist on my phone. Nor did any of its advertisements. My phone became the better part of the Facebook experience... without Facebook.
It doesn't surprise me in the least that this has come to light in the recent IPO. I gather that Facebook intends to start adding ads to news feeds. I wonder how long it will be before Facebook starts offering subscriptions to remove them. Or how long it will be before someone figures out how to filter them out again.
I have to say, though, the Facebook 2.5 update (relatively new) for Windows Phone does offer a couple features that make it worth keeping on the device.
The latest Facebook app can be found here: