That’s all beginning to change, with services like Stitcher and, to some extent, with Pandora’s new comedy programming — and it’s with good reason. Talk radio represents 35 percent of all terrestrial radio listening. It’s a $5 billion piece of the $15 billion radio advertising market. So why is this part of the market the last holdout of the Internet revolution? Why are we only beginning to see the familiar digital shift in talk radio now? For this segment of the market, unlike news, television, and music, Internet access isn’t the key to adoption — mobile Internet access is.