The mobile ad network AdMob has raised USD $12.5 million in a further round of venture funding.

The new investment is its third round of funding, which now totals USD $28.2 million.

Chief executive Omar Hamoui said the new funds will be used to make investments to ensure AdMob comes out of this "challenging economic environment" in a strong position.

The San Mateo, California-based company has made a number of announcements recently.

These include offering, first iPhone-customized ads and metrics, and then earlier this week a similar service for Google’s Android smartphone.

The new funding comes from Draper Fisher Jurvetson.

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