New York—U.S. mobile advertising revenue will reach $2.6 billion this year, up 74% over last year, according to eMarketer. By 2014, U.S. mobile ad revenue will top $6.6 billion, the research firm forecast. Google is projected to be this year's top mobile ad revenue producer, with $1.4 billion. A distant second is online music site Pandora ($226.4 million), while Twitter will rank third ($129.7 million). Facebook is sixth on the list ($72.7 million), after Millennial Media ($84.1 million) and Apple ($75.1 million).