From an article in the Washington Post as quoted on a private list by P.W. Fenton:

Ira Glass was recently involved in a sort of public Q&A session to
promote his latest “This American Life – Live” project and a fan asked
about podcasting…

Question: “Have you found that podcasting has changed TAL? I’m
incredibly grateful to the podcasts, which give me access to TAL for
my subway commute or running errands. I would imagine that there are
lots of people like me, and that TAL’s exposure has grown as a result
of podcasts. But are there other ways in which the podcasting has
changed the show?”

Ira Glass: “Interestingly, it’s pulled in a much younger audience. The
radio audience has stayed the same size – 1.8 million people a week –
but now there’s this extra half million people and they’re much much
younger than the public radio audience. Which is fantastic, of course.
I now meet lots of people in their teens and twenties at our live
events, and some of them aren’t public radio listeners at all. They
simply know us as a podcast.

In terms of content this hasn’t changed the show but in terms of
reach, it’s really nice.”