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Mobile ads are the Holy Grail of revenue for anyone with a social-media plan.

The market for the ads that dot smartphone and tablet screens is expected to soar to $10.8 billion in U.S. sales by 2016, from $2.6 billion expected this year, according to research firm eMarketer. That's a tiny slice of the $169.5 billion market for media ad spending in the U.S.

Yet mobile ads are crucial to the growth of many companies, including newly public Facebook, though few businesses have been able to capitalize on the promise.