On its first day of trading, mobile ad network Millennial Media's market cap far exceeded the amount advertisers will spend on the entire mobile banner-ad market in the U.S. coming year, reflecting the growth potential of advertising on smartphones and mobile devices.

Shares for the Baltimore-based mobile ad network were priced at $13 yesterday and almost doubled to $25 by market close. That makes the company, still operating at a loss, worth nearly $2 billion, which is more than twice the $861.7 million that advertisers are expected to spend on mobile display ads in the U.S. in 2012, according to eMarketer.