Microsoft’s Windows Phone mobile platform saw the highest click-through rates (CTR) for any mobile platform, both globally and in the United States in the third quarter, according to mobile advertising firm Smaato.

For the U.S., Smaato’s third-quarter 2011 metrics reveal Windows Phone (150) continues to lead in CTR closely followed by RIM (136).  Apple’s iOS finished fourth with a score of 93. Surprisingly, Symbian OS moved up two places to third, while Android (89) continued to lose ground for two consecutive quarters.

Ragnar Kruse, Smaato’s CEO and co-founder, said Windows Phone could see a larger audience going forward, given Microsoft’s new partnership with Nokia.

“Nokia and Microsoft have a great opportunity to establish a viable third ecosystem,” Kruse said in a statement. “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 & Asia.”

Kruse said developers should not 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.

The performance parameters in the Smaato Mobile Advertising Index are based on more than 40,000 registered publishers managed ad requests in the third quarter of 2011 and over 70 connected ad networks delivering mobile advertising across 230 countries.