First, there is the question of where Android's growth is. Apple is clearly going for the $200 price point on-contract. Yes, they sell their old models, but Android has a more diverse ecosystem. If we limited ourselves to equivalents (the top of the line iPhone and the several top of the line Android devices (Nexus 4, One X, GS III, etc.)) would there be so many more Android shipments? So, Android's shipments might be coming more on the low-end: the end that people like us don't purchase from as much.
Second, in the Mac vs. Windows era, there was a significant price disparity. If you're in the market for a top-of-the-line smartphone on-contract today, you're likely paying $200 for an Android phone or iPhone. Windows computers always had a significant price discount over equivalent Macs.
Third, in the Mac vs. Windows era, Macs often were under-spec'd. While Apple tried to make the best of the 603-G5 processors, Intel was just on a roll. You'd pay more and end up with less muscle. Today, the iPhone's A6 matches the quad-cores that Americans haven't been getting since LTE took precedence over quad-core in the GS III generation.
Fourth, in the Mac vs. PC era, Macs were technologically inferior for many years. I'm specifically talking about the Classic Mac OS. Mac OS 8/9 was beautiful. Windows looked like it was designed in crayon by comparison (to me). However, its cooperative multitasking was just sad. Windows 95 really showed that OSs could be better (even if I thought that its UI was sub-par). When OS X and Windows XP came out, OS X was dog-slow - slow to the point of unusable. Apple really improved it and made it into the OS that many of us (myself included) love today. However, I remember 2001 and how envious I was of Windows users.
Fifth, in the Mac vs. PC era, Macs were app-poor. There was just a lot of compelling stuff that you couldn't run as a Mac user. The move toward the web changed a lot of that and left Apple with a great way back into the market. With the iPhone, I find that the iPhone does a better job on web pages than Android (note, I haven't used Android 4.1 and 4.2). Stats on web usage on mobile phones seem to bear this out as iPhone web browsing tends to dominate Android web browsing. Likewise, the iPhone at least matches Android on apps, but I think it could be argued to be the primary platform for apps. So, there isn't a penalty to using an iPhone in the way that there was a penalty to using a Mac in the Mac vs. PC era.
Basically, the iPhone isn't more expensive at the top-of-the-line, the iPhone has specs that no one would deny are top of the line, iOS isn't an inferior OS (personal preference about certain features aside), and there isn't a penalty to using iOS (if anything, it could be considered the first platform). None of this is meant to say that the iPhone is better than an Android phone. I don't care what you enjoy using. It's meant to demonstrate that iOS doesn't have the same cost premium, that if we limit ourselves to the devices that people like us buy the marketshare disparity might not be there, that even if iOS is a minority platform it still gets as good attention or better from developers, the specs are equivalent which they weren't in computers for a long time, and iOS and Android are both great modern OSs. If you were an early adopter, iOS came a couple years before Android.
In the Mac vs. PC era, Windows was more popular not just because popularity breeds popularity. It was hugely cheaper, better spec'd, and a superior OS for a good while. If you loved computing, Apple's tidyness might have been well out-shadowed by Windows 95's multi-tasking, app availability, price, specs, etc. If you like iOS, why choose an Android device as your next phone? No reason really. Similarly, there aren't compelling reasons if you like Android to buy an iPhone - Android phones today have high-res displays, are starting to get serious on build quality, similar specs, etc. It wasn't hard to convince a Mac user in 2000 to switch to Windows: you got a better OS, more apps, a faster computer, for half the price. I could convince people to switch to Windows with good, logical arguments that really rang true. Today, how would you convince an iPhone user to switch to Android? I'm not saying Android is bad, just that it's a lot harder to make that argument.
I think there's another big reason: Facebook employees tended to be early adopters, and bought into smartphone back when iOS really was clearly the better platform. And then either got locked in to the ecosystem, developed brand loyalty, chose the path of least resistance, or just didn't notice when android became a worthy competitor. There are lots of good reasons to use an iPhone now if your first iPhone was four years ago. Fewer if your first smartphone is this year.
Step back and think here. Facebook has 800 million active users. Their major competition and growth opportunities will not be in the USA, they will be in China and emerging countries.
It's far from obvious in this bubble here, but these people are overwhelmingly not buying Apple devices. They simply can't afford them.
First, there is the question of where Android's growth is. Apple is clearly going for the $200 price point on-contract. Yes, they sell their old models, but Android has a more diverse ecosystem. If we limited ourselves to equivalents (the top of the line iPhone and the several top of the line Android devices (Nexus 4, One X, GS III, etc.)) would there be so many more Android shipments? So, Android's shipments might be coming more on the low-end: the end that people like us don't purchase from as much.
Second, in the Mac vs. Windows era, there was a significant price disparity. If you're in the market for a top-of-the-line smartphone on-contract today, you're likely paying $200 for an Android phone or iPhone. Windows computers always had a significant price discount over equivalent Macs.
Third, in the Mac vs. Windows era, Macs often were under-spec'd. While Apple tried to make the best of the 603-G5 processors, Intel was just on a roll. You'd pay more and end up with less muscle. Today, the iPhone's A6 matches the quad-cores that Americans haven't been getting since LTE took precedence over quad-core in the GS III generation.
Fourth, in the Mac vs. PC era, Macs were technologically inferior for many years. I'm specifically talking about the Classic Mac OS. Mac OS 8/9 was beautiful. Windows looked like it was designed in crayon by comparison (to me). However, its cooperative multitasking was just sad. Windows 95 really showed that OSs could be better (even if I thought that its UI was sub-par). When OS X and Windows XP came out, OS X was dog-slow - slow to the point of unusable. Apple really improved it and made it into the OS that many of us (myself included) love today. However, I remember 2001 and how envious I was of Windows users.
Fifth, in the Mac vs. PC era, Macs were app-poor. There was just a lot of compelling stuff that you couldn't run as a Mac user. The move toward the web changed a lot of that and left Apple with a great way back into the market. With the iPhone, I find that the iPhone does a better job on web pages than Android (note, I haven't used Android 4.1 and 4.2). Stats on web usage on mobile phones seem to bear this out as iPhone web browsing tends to dominate Android web browsing. Likewise, the iPhone at least matches Android on apps, but I think it could be argued to be the primary platform for apps. So, there isn't a penalty to using an iPhone in the way that there was a penalty to using a Mac in the Mac vs. PC era.
Basically, the iPhone isn't more expensive at the top-of-the-line, the iPhone has specs that no one would deny are top of the line, iOS isn't an inferior OS (personal preference about certain features aside), and there isn't a penalty to using iOS (if anything, it could be considered the first platform). None of this is meant to say that the iPhone is better than an Android phone. I don't care what you enjoy using. It's meant to demonstrate that iOS doesn't have the same cost premium, that if we limit ourselves to the devices that people like us buy the marketshare disparity might not be there, that even if iOS is a minority platform it still gets as good attention or better from developers, the specs are equivalent which they weren't in computers for a long time, and iOS and Android are both great modern OSs. If you were an early adopter, iOS came a couple years before Android.
In the Mac vs. PC era, Windows was more popular not just because popularity breeds popularity. It was hugely cheaper, better spec'd, and a superior OS for a good while. If you loved computing, Apple's tidyness might have been well out-shadowed by Windows 95's multi-tasking, app availability, price, specs, etc. If you like iOS, why choose an Android device as your next phone? No reason really. Similarly, there aren't compelling reasons if you like Android to buy an iPhone - Android phones today have high-res displays, are starting to get serious on build quality, similar specs, etc. It wasn't hard to convince a Mac user in 2000 to switch to Windows: you got a better OS, more apps, a faster computer, for half the price. I could convince people to switch to Windows with good, logical arguments that really rang true. Today, how would you convince an iPhone user to switch to Android? I'm not saying Android is bad, just that it's a lot harder to make that argument.