When Google buys a mobile advertising firm for $750M in stock, you know that mobile marketing is serious business. The planned acquisition of AdMob marks Google’s priciest buy since YouTube.

“Mobile advertising has enormous potential as a marketing medium and while this industry is still in the early stages of development, AdMob (see our past coverage) has already made exceptional progress in a very short time,” Susan Wojcicki, Google’s vice president of product management, said a statement.

Backed by Google investor Sequoia Capital, as well as Accel Partners, Draper Fisher Jurvetson and Northgate Capital, AdMob’s mobile marketing victory won’t be the last for startups in the space. The U.S. mobile ad market should reach $2 billion to $3 billion by 2013, up from less than $1 billion currently, according to Sanford C. Bernstein & Co.

AdMob, based in San Mateo, California, grew faster then CEO Omar Hamoui expected. Founded in 2006 in Hamoui’s dorm room at the University of Pennsylvania’s Wharton School, the advertising company grew into one focused on mobile display ads.

At the tail end of last month, AdMob released a report discussing smart phone adoption’s growth. The report highlighted the  growth of Google’s Android mobile 0S by 13% from August to September 2009.