Shares of Millennial Media, the second-largest mobile ad firm in the U.S., opened at $25 on the New York Stock Exchange this morning, up from an initial price of $13.

While its major competitors Quattro Wireless and AdMob were bought by Apple and Google, respectively, Millennial Media has stuck it out as an independent mobile ad company. And that seems to have paid off:  at $25 a share, Millennial’s valuation has increased to almost $2 billion. The company raised over $130 million with its initial offering.

The Baltimore, Maryland-based company makes it easy for brands and companies to advertise on mobile devices, and it also provides useful analytics about users to determine the effectiveness of campaigns. Millennial’s technology is cross-platform, unlike Apple’s struggling iAd platform (the remnants of Quattro Wireless).

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Millennial Media has yet to be profitable, but saw its revenue double last year to $103.7 million and its net loss improve to $287,000 (down from $7.1 million in 2010). The company’s clients include McDonald’s, Zynga, and Coca-Cola.

Via the Wall Street Journal; Image courtesy of emin kuliyev, Shutterstock