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Well what does this AdMob company do? , they serve ads for more than 9000 mobile websites and 3000 applications while they run the ads they store and analyzes data from all the ad requests, impressions and they use this to determine how they can optimize the ad network.
They also release monthly reports how each of the mobiles are doing in the chart.
Continue read bellow:

Highlights from this report:

• *Worldwide iPhone OS marketshare has increased from 33 to 40 percent in the last 6 months, with substantial share in all regions except for Asia and Africa. Please see our June report for more information on the geographic distribution of our iPhone users.
• Although its marketshare has been declining, Nokia continues to hold 12 of the top 20 smartphone devices in AdMob’s network. Nokia’s N97 and 5800 XpressMusic touchscreen devices were the fourth and fifth most popular smartphones in the UK in August.
• RIM smartphone share declined only slightly over the last 6 months. There are 7 RIM models in the top 20 smartphones in North America.
• Android is growing rapidly in North America and Western Europe. The HTC Magic (my Touch) is a Top 10 smartphone in both North America and Western Europe.
• Palm Pre also had a strong month due to the addition of new publishers to the AdMob network.