Podcasting's growth as a social-media tool hasn't yet matched expectations, but eMarketer thinks that may change in the coming years, thanks to Google.

eMarketer analyst James Belcher says spending on podcasting advertising will quintuple over the next five years, from a paltry $80 million in 2006 to $400 million in 2011, MediaWeek reports (via iMedia Connection). Belcher expects that by 2008 Google will have developed a version of AdSense that can be used with podcasts, allowing podcasters to add advertising to their casts.

While the podcasting market remains relatively small, podcasters and their audiences are a devoted group. This, coupled with low cost and highly targeted content, could prove a valuable medium for advertisers in the near future, Belcher says.

"These people constitute an excellent media target," Belcher said. "It's a self-selecting medium. These people are into it, and they're really into it."