Clear Channel’s digital sites have hit 22 million users per month and there are an additional 15% more listeners tuning in to stations online, over and above those who listen on AM and FM. The US radio and outdoor advertising company says this is a vindication of its focus on multi-platform content creation begun five years ago.

Clear Channel Radio’s active, unique monthly audience across all of its digital properties now numbers 22 million, according to data from Omniture, Inc. In addition to the raw numbers, the company is showing strong gains in year-on-year audience growth. There are now 850 online and on-demand streams, including niche music and genres, news, traffic, and other content.

For some stations, online and mobile phone listening now amounts to an additional 15% in total listeners beyond the AM/FM broadcast. Some of the company’s strongest gains are coming from its mobile platform.

To date, more than two million unique users have downloaded the iheartradio application. Just recently, iheartradio spent 10 days as the number 1 BlackBerry app and has remained in the top 20 since launch. For the iPhone and iPod Touch, the application consistently ranks in the top five among free music apps. Users of the application have also recorded longer time spent listening than most mobile aps.

Clear Channel Radio saw the number of advertisers choosing its digital platforms grow by 23% in 2008, to some 100,000 advertisers.