Worldwide mobile ad network fill rates averaged 10 percent in the third quarter of 2011, an 8 percent quarter-over-quarter decline, according to new data published by mobile ad platform Smaato.

The Smaato Mobile Advertising Index, which spans more than 70 networks delivering mobile ads across 230 countries, reports that fill rates across the top 40 networks ranged from 19 percent to 84 percent during the third quarter, reinforcing the argument that mobile inventory is increasing at a faster rate than mobile ad budgets. Thirteen of the top 20 U.S. mobile ad networks performed above the national fill rate average, which dipped from 19 percent in the second quarter of 2011 to 18 percent.

Looking at worldwide click-through rates, the Smaato Index (which sets the average CTR of all devices at 100) reports that Microsoft's Windows Phone led all operating systems in the third quarter at 156, followed by Symbian at 130 and Research In Motion's BlackBerry at 113. Apple's  iOS trails at 89, followed by Google's Android at 84. In the U.S., Windows Phone leads at 150, followed by BlackBerry (136), Symbian (129), iOS (93) and Android (81).

"It will be interesting to see how the new Windows Phone partnership will play out related to mobile advertising performance," said Smaato CEO and co-founder Ragnar Kruse in a statement. "Nokia and Microsoft have a great opportunity to establish a viable third ecosystem. They will have to establish a new niche ecosystem for the customer, however, the combination of security, established systems and a new look could be very popular in Europe and Asia. Developers are advised not to overlook the Nokia/Windows combination as it might be a platform that is easier to monetize, will have less threats of piracy as it will be a more closed ecosystem and will also offer more cohesiveness."