Mobile ad network AdMob has raised a $12.5 million addition to its latest round of venture funding, bringing its third round to $28.2 million in all.

In a blog post announcing the funding, chief executive Omar Hamoui says the new cash will help AdMob “cement our leadership position by making the investments that will help us to come out of this challenging economic environment even stronger than when we went in.” The San Mateo, Calif.-based company is already one of the biggest names in mobile advertising, and despite our concerns about how the economic turmoil would hit mobile ad spending, demand seems to be growing.

AdMob made a splash last year with its iPhone-customized ads and metrics. Earlier this week, it started offering similar services for Google’s Android smartphone.

The additional funding comes from Draper Fisher Jurvetson. AdMob really has a killer lineup of investors — Sequoia Capital and Accel Partners are both existing backers. AdMob has raised $47.2 million in all.

A company spokesperson tells me that AdMob has served more than 60 billion ad impressions, served more than 50 campaigns for brand advertisers in December, and has more than 6,000 mobile web sites and 450 applications in its network.

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