Monthly mobile ad requests across the worldwide AdMob network tripled from 1.6 billion in September 2007 to 5.1 billion in September 2008 according to the mobile advertising marketplace's latest Mobile Metrics Report. AdMob notes the surge can be attributed to a number of factors, including the global expansion of its network (from 16 international markets to the present 34), organic growth from legacy publishers and the addition of thousands of new mobile sites and applications. There are now 6,000 mobile sites and applications that make up the AdMob publisher network, with 4,308 publishers requesting ads last month. Other findings of the latest AdMob report: Apple's iPhone accounted for 103 million worldwide ad requests in September 2008, behind only the Motorola RAZR, Nokia N70 and Motorola KRZR. In the U.S., 16 of the current Top 20 devices made their retail debut from September 2007 onward, offering consumers the benefit of improved mobile web browsing, larger device screens, faster network connections and other next-gen enhancements. While worldwide ad requests fell 0.8 percent month-over-month to 5.1 billion, ads served increased 26 percent to 4.5 billion. In the U.S., requests grew 8.8 percent to 2.0 billion in September 2008. AdMob notes it implemented certain minimum thresholds and stopped serving ads in low performing inventory, resulting in the decline in worldwide requests. For more on AdMob's Mobile Metrics Report: - read this release Related articles: AdMob intros iPhone 3G advertising formats Mobile advertising making headway--sort of