Feedburner reports on the rising use of podcasting:

Podcast listenership may have a long way to go before catching up to the U.S. radio audience, but while radio audience is declining, podcast circulation is consistently growing nearly 20% per month. Today, there are more than 1.6 million aggregate subscribers to FeedBurner-managed podcasts, and this number has more than doubled in the past six months. Since we recently started tracking podcast downloads within feeds, we've also seen the ratio of downloads to subscribers average 2:1, suggesting a significant secondary market for podcast listenership beyond just the feed's subscribers. (For more on this, see "Uncommon Uses" below.)

Audience size runs the gamut. Some podcasts have a very large audience, like NPR's On The Media from WNYC, New York Public Radio, with more than 16,000 loyal subscribers (and as many as 40,000 who download media right from the Web site). Others are more personal podcasts created to keep friends and family up to date. While iTunes is the clear favorite for podcast subscribers, a healthy 43% of the market listens (or watches) their favorite podcasts using other applications. That said, the two most significant jumps in podcast circulation in the past 18 months can both be tied to iTunes: In July, when iTunes 4.9 launched with podcasting support, and the end of last year, when millions of people unwrapped an iPod (Apple sold 14 million iPods in Q4 last year).