It was not that long ago that podcasts were quite popular. The proliferation of portable audio devices like early iPods and Zune devices helped to create a demand for audio that spilled into a podcast revolution.
According to Wikipedia, the mid-2000s were a heyday for podcasting with a growth in interest spurred by people who liked radio but found an ability to customize the programming and the ability to listen on demand.
But there is an interesting part of the podcasting story that is left to be told. Would you believe they are still quite popular? According to the Pew Internet and American Life Project, one in five people online are listening to podcasts. That is nearly twice as often as people use Twitter.