Seems odd, but more than 18% of internet users visit radio sites each month and listen to programming while they are there. According to Media Daily News:
ALMOST ONE-FIFTH OF AMERICANS ON the Internet visit the Web site of a radio station at least once a month, according to market research from the Media Audit released on Thursday. With 70% of the total American population on the Internet–about 212 million people–that equals roughly 38.6 million unique visitors a month. That’s good news for radio broadcasters looking to open up new revenue streams online.
A few radio behemoths have moved quickly to monetize these streams of new visitors.