iPhone is bigger than all of Microsoft

Apple originally didn't allow third-party apps on the iPhone. This year, Apple's iTunes App Store is on pace to generate $4.5 billion in revenue for developers, according to IHS iSuppli. Apple takes home a 30% cut of those sales.

Opening up its operating system to app makers turned out to be a pretty good idea.

It didn't take long for the third-party app model to blossom. Users downloaded just 10 million apps in 2008, the first year that Apple opened up its smartphone to third-party apps. But the downloads kept on adding up. There have now been 30 billion apps downloaded since the iPhone launched its App Store four years ago.

Apple currently has 650,000 apps available in its App Store, the most for any mobile platform.

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