Mobile advertising will grow from 18% to 23% of revenue generated by iOS and Android apps, according to a new analysis by app advertising and analytics firm Flurry. The firm forecasts that app ad revenue will more than double in 2012 to $2 billion. The bulk of the $8.7 billion in projected revenue this year, up 60% from 2011, will still come from paid apps and in-app purchases. The Flurry study also predicts long-tail apps will account for a growing share of app revenue. To reach that conclusion, the company compared app rankings on iOS and Android in 2010 and 2012 in three segments: the top 25; those ranked 26-100; and those below the top 100.