'Barriers to entry'
For Michael Wolf, a market analyst and podcaster himself, it's no surprise in the sudden increase in interest in the podcast.
"The barriers to entry have come down," he told CNBC, highlighting how smartphones have made it easy to subscribe to and download podcasts. "And the biggest names in the business have really started to innovate."
Because "Serial" isn't the only popular podcast. Other series with massive audiences include "The Adam Carolla Show", Stephen Dubner's "Freakonomics" and Roman Mars' design podcast "99% Invisible". Mars' show proved so popular that he quit his job on public radio to work full-time on the podcast—which is funded solely by fans and sponsors.
Because most podcasts are a one-man operation, the hosts often also read out their sponsors' messages.
For these sponsors, podcasts offer something different to more traditional media, according to Adam Sachs, CEO of podcast advertising network Midroll Media, which specializes in host-read ads.
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"We know that podcasts are a very intimate medium – the hosts are literally in your ear on a regular basis," he told CNBC. "They feel like your friends. So it can be very powerful when they read an ad."
Podcast sponsors have traditionally been technology companies appealing to a tech-savvy listener. The sponsor of "Serial", for instance, is Mail Chimp – an email marketing service provider.
"Podcasts' listeners are online—maybe they're creating their own websites. You can see why it appeals to Mail Chimp," Wolf said. But this could be about to change.
Sachs said non-tech companies such as Random House and HBO were currently among the company's 250-plus advertisers. "We are seeing interest from more and more bigger brands."
And advertiser interest has increased since "Serial" was released, Sachs added. "It's made podcasting more mainstream. In the U.S. at least, I'd say it's talked about more than any TV show," he said. "That's attractive to advertisers."
In fact, Mail Chimp's sponsorship of "Serial" has spawned its own meme and Twitter hashtag - #mailkimp – based on a mispronunciation of the company's name in the pre-roll ad. Online news service Quartz called it "the year's biggest marketing win".
For Edison Research's Rosin, what was "hugely significant" about "Serial" was the interest from "Hollywood and the creative classes". That, he said, is where the innovation – and money – was.
"Podcasting was a germinating form of content that was waiting for a hit. It finally has one," Rosin said.