Control, privacy, platform, and competition are just some of the reasons both iOS and OS X integrate Twitter but not Facebook.
With the upcoming release of OS X Mountain Lion, Apple will be fully integrating Twitter social sharing across all of their included apps, as well as extending a developer API for 3rd party apps available through the Mac App Store. This follows similar integration in iOS 5 last year and leaves one huge, lingering question: where's the Facebook integration?
Facebook has roughly 845 million users on their platform, and Facebook for iPhone has been one of the most popular apps on the App Store since launch. Both iOS and Facebook are massive platforms and the intersection of those to platforms is enormous.
That's not taking anything away from Twitter, which is also extremely popular and also shares a huge intersection between its user base and Apple's. They're just very different social networks, with different features, and iOS currently only has one and not the other.
So why would Apple choose to leave out Facebook and rely only on the less-enormous, less feature-filled Twitter for social integration in OS X 10.8 and iOS 5?
Control
The first reason that springs to mind is control. Apple likes to have as much control over their platform as they can in order to deliver the absolute best user-experience possible. Twitter doesn't take much issue with this, but Facebook is a different story.
Apple and Facebook once tried to work together on Apple's Ping social music service. According to the late Steve Jobs, Facebook wanted "onerous" terms in order to allow integration between Ping and Facebook's platform. Apple declined. Facebook pulled support. Finger pointing and harsh words followed.
(There are also some signs that Apple and Facebook flirted with integration in iOS 4 as well, but nothing became of it.)
Privacy
Apple has had some issues with privacy, including the collection of traffic data and recently apps that uploaded Contacts info without permission. However, they've also used privacy as a way to needles Google. Apple makes most of their money selling products, not selling advertising services, so they don't want to or need to collect a lot of user data. They've also been fairly unimpeachable when it's come to insisting other companies get users to opt-into sharing information up front, rather than force them to opt-out in some convoluted manner later. (Much to the consternation of advertisers and marketers, most recently in the magazine subscription area.)
Facebook on the other hand likes to collect huge amounts of data from its users, often with a collect first, apologize later mentality. Apple might take issue with Facebook wanting to harvest iOS user data, package it up, and market against it. Especially if all they, and users get in return is integration for status, photos, and location.
Platform
Facebook views their service as a platform. Just as OS X and iOS are unique and proprietary platforms meant to help Apple sell hardware, Facebook's platform is a proprietary service meant to help them collect data and sell ads. Apple wants to prioritize their iPhone and iPad devices, Facebook their social graph.
Those different and diverging priorities can easily be at odds and lead to conflict.
Competition
Lastly, Facebook has long been rumored to be working on their own smartphone. This could present the same problem for Apple that Google did when Android was released. In fact, it could be an even bigger problem if Facebook's integration is more broad than a maps or video app, or a mail account. Facebook integration isn't a bell that can be easily un-rung.
It would make sense for Facebook to make their own smartphone, however, the same way it made sense for Google.
Steve Jobs threatened to go Nuclear on Google when Android was released, to spend Apple's last dollar suing Google over their "stolen" technology. Would Apple set themselves up for that to happen again?
Twitter doesn't seem to want as much control as Facebook. They're so far much more upfront about user privacy. While they're also a platform, it's one that seems -- at least for now -- more compatible with Apple's needs than is Facebook's. There's also no sign of a Twitter Phone on the horizon (or rather, almost every smartphone is already a Twitter Phone these days.)
Of course, it's always possible that Apple and Facebook may come to some sort of agreement down the road, ultimately paving the way for Facebook integration in iOS 6 and future versions of OS X. During Apple's recent shareholder meeting, CEO Tim Cook was asked about their relationship with Facebook and where exactly the popular social network would come into the picture down the road.
We do a lot with them, our users use Facebook an enormous amount. [...] I've always thought that the two companies could do more together.
Apple could stay away from Facebook just as they have always done, allowing Twitter to continue on its course of massive growth and user adoption. Or Apple and Facebook could patch up their differences, work out a deal, and give users more benefit with additional social sharing options..
Regardless, it would be nice to see the two work something out given how popular Facebook is and how often most of us use the social network on our iPhones, iPod touches, and iPads.
