In all the recent discussion about podcasting having arrived — with a lot of reliance on Edison Research's most recent in a multi-year series of interviews about radio and audio-program listening — nobody seems to have connected two numbers.
The Pew Research Internet Project's Social Networking Fact Sheet pegs regular Twitter use at 19% of online adult Americans. That's people 18 or older who use the Internet regularly, which Pew says elsewhere is about 87% of adults.
Edison Research's lengthy research deck notes that of its panel, which includes Americans aged 12 and older, so a larger pool, 15% had listened to one or more podcasts in the previous month when the survey was conducted. (Edison redefined how it counts this in 2013, which means the 9% figure in 2008 might be overstated by current terms, but helps show a slow, steady rise over the last six years.)
Edison helpfully notes that the 15% figure represents 39 million Americans. There are 309 million people in America as of the 2010 census, and the Census Bureau says 23.3% of them are not yet 18 years of age. Thus 76.7% of 309 million gets you 236 million; take 87% of that for online adult Americans, and it's 205 million; 19% who use Twitter regularly would thus mean…39 million.
A podcast and Twitter aren't comparable in nature. Someone might listen to one 30-minute episode of a radio show or their local church's sermon once a month, while they participate on Twitter every day; the opposite is also possible.
Yet given the attention paid to Twitter, it's reasonable to think that podcasting quietly arrived at a viable mass market when no one was looking. It took Serial for people outside of radio and podcasting to pay close attention.
Twitter's growth has slowed, especially for active users. Podcasting has by no means reached its top, and it's likely to be driven higher by a critical mass of adoption and shows like Serial. The number of podcast listeners could start to approach Edison's figures for online radio listeners: about 47% of the 12+ population in America, or about 124 million people.
For people who love listening to and making podcasts, 39 million is a very nice potential audience, but striving towards 124 million sounds even better.