FeedBurner works with 40,000 Web content publishers to make it easy to subscribe to audio and RSS feeds. Since November, when the company pointed to 505 podcasts, the number has now grown to almost 6,000.

The co-founder of the Chicago-based, venture capital-funded firm revealed statistics about the popularity of the audio programming meant to be downloaded to PCs or mobile device players.

“The average number of subscribers to the shows we manage was 15 last year. Now it’s 33,” he said. “These are pioneer days.” Lest you dismiss those numbers, however, “Our 20 highly popular shows have thousands of subscribers and our couple of top podcasts have tens of thousands.”

He said the number of subscribers to podcasts has grown by “an order of magnitude from just three months ago.”

Costolo declined to identify the most popular programs, but said “in the very near future” FeedBurner publishers may start releasing circulation numbers. Adam Curry has claimed 50,000 subscribers to his Daily Source Code. The producer of “Behind the Scenes,” a podcast about podcasting, reports 3,000 to 5,000 listeners per show.

Costolo said podcasting is, “Clearly something that’s taking off,” and eventually it will be turned into a money-making business. “Business and commerce opportunities will expose themselves,” he said, adding, “Now it’s important to make it easy to create and subscribe and distribute podcasts. Then the economics will take care of themselves.” Listen to an interview with FeedBurner’s Costolo.

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