That's according to the latest data from Nielsen Mobile, which puts MSN second.
Google's mobile services had 13.4 million unique users in December 2007, followed by MSN with 11.1 uniques and AOL with eight million.
Data released today from Nielsen Mobile also shows that there were 35.6 million unique users of all mobile web services in the US in December, up from 31.9 million in August.
The audience measurement specialist also broke out some stats on mobile advertising, including 57.7 million US mobile users recalling seeing ads in 4Q 2007, equivalent to 23/3 per cent penetration. That's up from 42.4 million and 17.9 per cent penetration in 2Q 2007.
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