From a 21 Feb. 2007 press release:
The new semi-annual study from Bridge Ratings & Research indicates the number of monthly Internet radio* listeners nationwide has jumped 26% over last year and has increased to 72 million monthly listeners from 45 million at the end of 2005. ¶ In 2005, weekly Internet radio listening was at 15% of the U.S. population 12 and over. This new study shows that as of January 2007 that figure increased 26% to 19% of all persons 12 and older. This translates to 57 million listening to Internet Radio on a weekly basis. See latest digital projections. ¶ Last year's study revealed that 16% of those on-line had listened to an AM/FM simulcast on the Internet. This study, completed in January 2007 with a sample of 3000 persons 12+, showed 21% having had listened to an AM/FM simulcast in the last 30 days. ...
Link: Bridge Ratings. By way of comparison, on a weekly basis, the number of Internet radio listeners is roughly double the number of NPR listeners.
Updated 1 Mar. 2007: Fixed link under "See latest projections" above. --Dennis