How Many Podcast Listeners in USA

Podcast has been slowly and steadily getting mainstream in the USA, and the number of podcast listeners as percentage of internet users has been growing. Here is the projection of podcast audience as percent of internet users from 2008 through 2013.

2008 : 9%
2009 : 11%
2010 : 13%
2011 : 14.5%
2012 : 16%
2013 : 17%