Google's (GOOG) Android platform continued to gain smartphone market share in the U.S. in 2012, according to the results from a recent comScore survey.

Android now owns 50.1% of the market, up from 46.9% a year earlier. Even Apple's (AAPL) iOS operating system saw market share increase to 30.2% from 28.7%, primarily due to continued high sales of the iPhone 4S.

The race is now clearly a two-horse one, with other rival smartphone systems losing share in the same period. Research In Motion's (RIMM) BlackBerry share declined nearly 3.2% and Microsoft's (MSFT) Windows Phone and Windows Mobile combined market share fell by 1.3%. (Microsoft owns and publishes Top Stocks, an MSN Money site.)

Mobile advertising accounts for around a third of Google's total value. Google is betting on Android to drive mobile ad revenue in the coming years as smartphone and tablet penetration increases, and advertisers increasingly use the mobile advertising route to reach potential customers. Since Google, ironically, generates higher average advertising revenue from iOS devices, it benefits from an increase in mobile internet usage driven by higher sales of iOS devices as well as Android devices.

We currently have a $670 Trefis price estimate for Google, which stands nearly 4% above its market price.