What's wrong with tracking raw downloads? This seems like a great way to measure a podcast's success since the number of downloads probably tracks pretty closely to the number of listeners (sure, I might download an episode or two of a new podcast to try it out, but I generally only download later episodes of podcasts I actually listen to).
As an aside, I think that podcasts are to radio what netflix is to television, in that netflix is able to profit off of shows with niche audiences. With users pulling content rather than producers pushing content, you end up with a lot more niche content. In the past month alone, I've listened to podcasts about:
* Analysis of supreme court decisions (First Mondays)
* A history of the watergate scandal (Slow Burn)
* A history of the English Revolution (Revolutions)
* Political interviews (various podcasts)
* Improvisational comedy (Improv4Humans)
* Random historical figures with a comedic twist (The Dollop)
Some of what I listen to is meant for a mass audience, but some of it is targeting a pretty specific audience, which I think is awesome. Personally, I love finding podcasts that get into the nitty-gritty details of whatever the subject is. I almost don't care about what the subject is, as long as the presenter is passionate about it.