facebook phoneMark Zuckerberg nearly avoided mentioning the iPhone in last week’s announcement of Facebook Home, the application launcher and homescreen replacement tool that puts Facebook (and messaging) at the forefront of supported Android devices. If it weren’t for pesky, pestering reporters asking about Home coming to iOS, Zuckerberg likely would have skipped over the iPhone entirely, allowing Facebook to pretend that the world’s best-selling, least-social smartphone doesn’t exist.

The iPhone is, in many ways, the opposite of Facebook Home and Mark Zuckerberg’s vision for mobile computing. Home is a free application launcher that exists largely because of Google’s commitment to an open Android; Apple builds the iPhone and iOS for itself. Zuckerberg wants to refocus computing around people instead of software and, in the process, replace nearly everything about a smartphone with Facebook-built tools; Apple remains focused on software over social and seems content with its six-year-old vision for what a smartphone interface should look like.

And then there’s the fact that Apple has been slow to adopt social features within iOS, lagging years behind Android and its hyper-connectedness. The company didn’t introduce social features into iOS until it baked Twitter into iOS 5 — Facebook didn’t make its way in until a year later, with iOS 6. Even that integration is divorced from the iPhone’s main interface, restricted to parts of the operating system that average users may not even notice. (Share Sheets, anyone?)

Facebook wants to be the most important thing on its users’ smartphones. Home facilitates that goal by allowing the company to inject itself into every interaction a user has with his smartphone. The iPhone doesn’t allow for that, and Apple would probably prefer that its own applications, iTunes, and the App Store stay in front of users’ eyeballs.

“Companies grow,” writes the New Yorker’s Matt Buchanan. “Ideas, and Facebook, spread.” The social network attracted 1 billion users by first going for Ivy League students, then students from other colleges, and then, eventually, everybody; it probably thought it could jump from device to device in much the same way, first grabbing the Web, then smartphones, but that hasn’t been the case.

Apple has prevented Facebook from spreading in the same way it has on other platforms (and would almost certainly never allow Facebook to completely replace the iPhone’s user interface), stopping the social network from consuming the world’s most popular smartphone.

So, while some could view Home as a threat to Apple, both because it is more social (and, the thinking goes, more likely to catch on amongst Facebook-addicted consumers) and because it unifies Android devices in a previously-unseen manner, it seems that Home could just as easily have been borne from Facebook’s failure to dominate the iPhone.

Now there are (potentially) three categories of devices that truly matter in the mobile market: Android smartphones, Android smartphones dominated by Facebook Home, and the iPhone. Facebook could spread to every Android smartphone in the world and it still wouldn’t have the iPhone. While most companies would call that a success — when was the last time you heard of a piece of software being installed on every device on the planet? — Facebook could well see that as a failure.

Again, Facebook wants to become the most important aspect of any smartphone to its 1 billion users. That can’t happen as long as Apple continues restrict what Facebook is able to do with the iPhone. Ideas spread. Facebook spreads. And so long as the iPhone remains Home-free, the idea that Facebook can replace traditionally software-driven interfaces will be limited in its reach.