Time spent listening to online music radio was up 26% last year, with the segment logging 4.85 billion listening hours in 2007, according to a research report out from AccuStream iMedia Research.

AccuStream's "Net Music Radio 2007-2010: Listening Hour Analysis by Site and Brand" reports that total listening hours averaged 404.2 million hours a month in 2007, up from an average of 320.5 hours in 2006. That includes time spent listening to subscription music services Napster, Yahoo Music and Rhapsody.

AOL's Shoutcast was the top radio destination with 48.4% of the listening audience followed by Clear Channel Online, Yahoo Music, AOL Radio Networks and Pandora.

Internet music radio ad billings for audio ads more than tripled in 2007 to $80 million, up from $26 million the previous year. The segment generated an additional $12-$15 million from video ads.

AccuStream predicts that the segment could grow to $525 million in annual revenues once it fully matures.

The report also found that online radio visibility has improved partly based on online efforts by traditional radio companies Clear Channel and Citadel Broadcasting.