Flappy Bird, the hit mobile game, was -- until last week -- only available for Apple's (AAPL 0.47%) iOS. Facebook (META 0.27%) Paper, the new app from the social networking giant, remains an Apple exclusive for the time being.

Although Apple's iPhone has long lost the global market share battle to Google's (GOOGL 0.07%) Android, iOS remains the premiere platform for mobile developers. While that remains true, Apple shareholders have nothing to worry about.

Developers still favor iOS
Flappy Bird and Facebook Paper are just two examples of mobile developers prioritizing Apple's platform. There are countless others -- from Tinder to Mailbox to Instagram -- but Flappy Bird and Facebook are particularly notable.

What stands out about Flappy Bird, the mobile game that seems to be taking the world by storm, was the country where it originates: Vietnam. Not that I have anything against Vietnamese video game creators, but it's interesting that a game studio based in a developing market would favor iOS over Android.

According to IDC, Google's mobile operating system absolutely dominates Vietnam -- more than 82% of the smartphones shipped to Vietnam in the second quarter of last year ran some form of Google's operating system, while Apple's iPhones represented fewer than 2% of the smartphones in the country.

Facebook, in contrast, is a $150 billion corporation in California, with more than enough resources to develop for any platform it chooses. Facebook is no stranger to Android, launching Facebook Home on Google's platform last year. Still, it hasn't yet ported Paper to Android.

Unlike Facebook Home (which wouldn't have been possible on Apple's platform), there's nothing to stop Facebook from bringing Paper to Android. I'm sure it will in time, given all the rave reviews it's received, but for now, it hasn't bothered. According to TechRadar, Facebook Paper was the project of a small, internal dev team -- one with limited resources. It's hardly surprising, but still quite telling, that they chose to invest their limited resources in Apple's iOS.

There's danger in dwindling market share
Yet, longer term, Apple shareholders should still be concerned about the company's market share. Apple's domination of developed markets (particularly the US and Japan) gives it enough pull to entice developers -- for now.

But as other markets grow in importance, the dynamics of the app economy could shift, putting Apple at a severe disadvantage. It should be noted that Facebook Paper remains a US exclusive for the time being. If Facebook decides to take the app overseas, I'm sure it will be in the form of an Android version. Other app creators, who've wanted to market to consumers around the world, have been forced to release for Android -- in the US, Plants vs Zombies 2 wasn't released on Android until last October; Chinese gamers with Android handsets could play it in July.

Having access to apps, and having access to them first, is a key selling point of Apple's platform. As long as it continues to receive the best apps before Google's Android, Apple shareholders need not fear iOS becoming marginalized by Google's massive platform.

But if that trend ever changes -- watch out.