35% of the ads on the top 100 podcasts were for 5 companies

"Midroll sells ads for over 200 podcasts, including some of the most popular ones like WTF with Marc Maron and the Bill Simmons Podcast. The not-so-secret reality about podcast ads, though, are that advertisers are quite concentrated: a FiveThirtyEight intern heroically listened to the top 100 shows on the iTunes chart and counted 186 ads; 35 percent of them were from five companies. More tellingly, nearly all of the ads were of the direct marketing variety."

Source:  Stratechery, 7th June 2016

Full FiveThirtyEight piece is here, with more stats