By Prince McLean

Published: 07:25 PM EST (04:25 PM PST)

A new report by Google's AdMob subsidiary details that the installed base of iPhone OS devices visible on its ad network in April is twice as large as Android in the US, and 3.5 times larger globally.

The metrics look only at devices that have made a request to the AdMob network, so they don't count people who have not used any of the 23,000 apps or websites making use of AdMob's display ads within the test period.

"These unique device numbers are from the AdMob network only and reflect the adoption of our products and business operation," the report states. "We don’t know what percentage of the total universe of iPhone and Android devices AdMob reaches. However we believe the data is useful on a relative basis given the large sample size of devices in our network."

Broken, blended or buried?

Apple reports having sold more than 85 million iPhone OS devices while AdMob only sees 40.8 million worldwide, or less than half of those ever sold within its transaction logs.

Some number of those devices have been taken out of service, but if only half of those sold are currently being used, this suggests an extremely short lifespan for mobile devices. This also implies Apple will sell a tremendous number of new devices once the new iPhone 4 is released.

iPod touch and iPad in the App Store installed base

The numbers also indicate the power of Apple's iPod touch (and increasingly iPad) to dramatically boost the installed base of iPhone OS devices demanding software.

If roughly 80% of the active installed base of all iPhone OS devices in the US are iPod touches and iPads, that means even if Android phone sales catch up the iPhone (as they did in the US in the first calendar quarter of this year) Apple will still maintain a much larger market for developers to target.

Global distribution

More than half of Apple's sales are now outside the US. AdMob's figures indicate 49% of the iPhone OS devices reaching its ad network are in North America, while 28% are in Western Europe, 14% are in Asia and nearly 10% are elsewhere around the world.

For Android, a whopping 75% of the installed base visible to AdMob are in North America, with only 11% finding their way to Western Europe. A similar 12% are found in Asia, while less than 3% are in use elsewhere on the globe.

AdMob reports that the iPhone remains its top smartphone worldwide, making 41.6% of the ad network's total requests. However, that number is down incrementally from January when it represented 50% of the total ad traffic. The second place model is the Motorola Droid at 7.3%, followed by three HTC models that each claim less than 3% traffic share.

In the US, the iPhone has shrunk from 47% ad share in January to 38% share in the most recent report, while the second place Droid is at 16.1%. Overall, Android has grown from consuming 39% of the network's ads to 46%, making Android bigger as an ad base than the iPhone in this country.

In the UK, the iPhone maintains a whopping 63.9% share of the smartphone ad market, although that figure is also down from 79% in January. Other countries remain dominated by Nokia's Symbian phones, including India, Indonesia, and the Philippines, where Symbian makes up more than 90% of the ad market among smartphones.