Google is on pace to generate 2012 mobile advertising revenues in excess of $4 billion, according to a new forecast issued by Piper Jaffray analyst Gene Munster.

Basing his estimates on Google's growing footprint across its own Android mobile operating system as well as Apple's rival iOS, Munster forecasts the company's mobile ad business will grow from $2.5 billion in 2011 revenues and $1 billion a year earlier. "On the advertising side, one source shows that Android is doing well as a percentage of mobile ad impressions on their network, which we believe makes sense since a greater portion of Android apps are free versus paid," Munster writes in a Jan. 6 research note. He adds that with Android now topping 50 percent market share worldwide, the OS "ultimately becomes the biggest platform for mobile advertising both for Google and the industry."

Research firm IDC reported late last year that Google's AdMob controls 24 percent of the mobile advertising segment, ahead of independent Millennial Media at 17 percent and Apple's iAd at 15 percent. Google generated nearly $40 billion in digital ad sales in 2011 across all platforms.

Google senior vice president of mobile Andy Rubin recently announced the company activates more than 700,000 new Android-powered devices each day. Last week, app marketplace analytics firm Distimo reported that Google's Android Market now offers more than 400,000 applications, up from 300,000 in August 2011.

Munster adds that marketer interest in the mobile channel continues to increase. "Mobile now accounts for 10.1 percent of time spent with media, up from 5.4 percent in 2008, but only 0.9 percent of advertising dollars," he writes. "The shift to mobile will likely come at the expense of other over-indexed media like newspapers and magazines, but as we know from the shift to online advertising, the shift is likely to be slow and steady."