It’s been a year since Google (GOOG) released Android OS, the open-source smartphone operating system widely perceived as the most likely to overtake Apple’s (AAPL) iPhone in the long run.

As it happens, Google this month also purchased AdMob, the world’s largest purveyor of mobile phone advertising. So this seemed as good a time as any to take a snapshot of the changing smartphone marketplace, as measured by ad requests to AdMob’s network.

We reviewed a year’s worth of AdMob data — including the October numbers released Monday — and charted it on the graph at right (reproduced full-size below the fold).

There’s a bias in the data, since AdMob ads run better on iPhone OS and Android devices than on, say, Research in Motion (RIMM) BlackBerries. But the trends are clear.

Over the past year, Nokia’s (NOK) Symbian has lost the largest raw market share, down to 25% last month from 59% the same month a year earlier. In percentage terms, Windows Mobile (MSFT) is the biggest loser, down 70% in 12 months, with Symbian, Palm’s (PALM) Web OS and BlackBerry OS close behind.

These numbers are based on worldwide ad requests. Apple’s lead is even greater when AdMob zeroes in on the U.S. and U.K. markets. For a look at how the iPhone’s share of the U.S. and worldwide markets have grown, see the chart prepared by MacRumors’ Erik Slivka here.

Below the fold: A full-size fever chart of AdMob’s worldwide data for all the major smartphone operating systems.

[Follow Philip Elmer-DeWitt on Twitter @philiped]