On the other hand….

Pete Davies:

“This post explains why we have no idea how many people are actually listening to Serial (or any podcasts).”

In short, because there is a very real difference between downloading a podcast and listening to it. And many people will download the same episode multiple times (I’ll often have the same episode of something on my phone and my computer – and sometimes I’ll never wind up listening to it). A bit of caution on top of all the hype.
Another bit of caution:

“Our culture industry generates an infinite supply of Mad Men recaps and Serial thinkpieces, even though the audiences for those shows would be a rounding error for, say, NCIS or The Big Bang Theory.”

Podcasts may be booming, but they’re hardly mainstream.