Mobile advertising is big business and if you need any evidence to support that claim, look no further than YouTube. That's because the company has tripled their mobile ad sales in the last six months which could be worth as much at $350 million.

That's impressive growth but naturally there's a bit of back story behind how it all happened. According to Phil Farhi, director of product management at YouTube, the growth wasn't based on a decision they made but rather one from Apple.

You see, last summer Cupertino revealed that their license to include a YouTube app with iOS had ended. As such, iOS users were invited to either watch YouTube in the Safari browser or wait until Google released a YouTube app in the App Store.

That's exactly what Google did, releasing a free ad-support YouTube app for iOS devices. The app, along with similar versions that come loaded on Android devices, shows advertising in the form of commercials before some clips play. And the rest, as they say, is history.

In hindsight, it would seem that Google made a wise decision in going with an ad-supported version of the YouTube app. That's especially true when you consider a recent ComScore report showed Apple still holds a 39.2 percent share of the US smartphone market. Data from Nielsen Holdings revealed that at least 70 million people in the US used the app in March which is an increase of 42 percent year over year.