• by Mark Walsh  @markfwal, August 12, 2011

In March 2010, Apple's iOS platform accounted for 70% of ad impressions on Millennial Media's mobile ad network. As of the second quarter, that commanding share has dwindled to 27%, while Google's rival Android platform has surged to claim 54% of impressions. 

That finding is among the dramatic shifts unfolding in mobile advertising in the last two years, as highlighted in Millennial's  latest monthly metrics report -- its 50th since March 2009. What else has changed? While Android's rise has come at the expense of iOS, the iPhone itself has only increased its dominance as a mobile advertising vehicle. Its share among devices, based on impressions, has jumped from almost 6% to 16% in the last two years.

At the same time, no other individual handset accounts for more than 5.5% of impressions -- about the same as in 2009. In addition to the iPhone, the BlackBerry Curve is the only other smartphone that is still among the top 20 on Millennial's network. Almost all the rest are now Android phones.

Thanks in part to the iPad's arrival last year, Apple has also increased its share as a device manufacturer, from 11% to 30% of impressions. The iPod Touch, iPad and Samsung Galaxy Tab were the top three connected devices on the network in the second quarter. The spread of smartphones and tablets in the last two years has also pushed touchscreen devices from just a third of devices to 60%.

The new Millennial report also underscored how marketers have become more sophisticated in targeting mobile campaigns. Whereas only 1% of campaigns were geared to local markets in 2009, for example, 20% used local targeting in the second quarter of this year. Likewise, demographic targeting increased from 1% to 19% of campaigns.

The use of various calls to action in mobile ads has expanded rapidly. The proportion of campaigns including a link to a mobile site in ads has increased from 15% to more than half (52%), while click-to-call buttons have risen from just 1% to 37% of campaigns. And nearly a quarter (23%) of campaigns aim to drive app downloads, compared to 3% two years ago.

Millennial said the fastest-growing ad categories by spending in the second quarter compared to the prior quarter were finance, retail and restaurants, pharmaceuticals, automotive, entertainment and travel. Spending in each segment is up by at least triple digits. Retail and restaurants, telecommunications, and finance were the top categories by spend in the U.S, while portals and directories, education, and travel were the biggest internationally.

In June, Gartner released a forecast projecting mobile advertising revenue would double to $3.3 billion worldwide in 2011, and grow to $701.7 from 304.3 million in 2010 in North America. Research firm eMarketer has predicted U.S. mobile ad spending will reach $1.1 billion this year.