updated

admob.bmpAdMob, which says it has the largest ad platform for mobile phones, has raised $15 million in a second round of financing led by Silicon Valley’s venture firm Accel Partners, and including existing investor Sequoia Capital.

AdMob, of San Mateo, Calif., has served more than 1.6 billion ads in less than a year, and says that is more than any other player, including Third Screen Media, a Boston-based competitor that AOL is reportedly in talks to acquire (see WSJ).

But it isn’t clear whether AdMob is king. It says it served more than 600 million ads in Feb and March combined, an impressive acceleration from the 1 billion ads during the entire second half of last year. However, Third Screen says it shows about 350 million ads every month (we haven’t confirmed this, but this is what it tells the WSJ), suggesting it may be bigger. Third Screen has also announced major customers, from Fox News to Ford Motor Co. AdMob, by contrast, serves smaller companies.

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[Update: We wonder if anyone is holding the companies to the fire on their stats. We’ve just seen a reference at Third Screen’s own site to a mere 175 million impressions served.]

The mobile ad trend is just taking off. AdMob has 1200 publishers using it, across 160 countries. Despite this, it isn’t profitable yet. AdMobs lets advertisers target things like geography, behavior, demographics and handset manufacturer.

Other mobile ad start-ups include Ad Infuse, Enpocket, Millennial Media and Rhythm NewMedia. Google and Yahoo, the ad giants of Silicon Valley, serve ads alongside their mobile search ads, but have yet to roll out more general mobile ads.

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