Internet audio service Pandora says its share of total U.S. radio listening for June was 5.98%, an increase of 77% from 605 million during the same period last year.  

Listener hours for Pandora during the month of June were 1.08 billion, up 77% from 605 million in June 2011. The company says it had 54.5 million active listeners at the end of this June, an increase of 51% from 36.1 million during the same time period last year.

While Pandora has been releasing key audience metrics, the company is also laying the groundwork to get its royalty rate cut when its current agreement runs out in 2015.

We recently reported that Pandora is paying approximately 50% of total revenues in music royalties; That rate is unfair compared to other audio media like satellite radio, for example, according to company President Tim Westergren.

Pandora has ramped up its Washington lobbying efforts to lay the groundwork with lawmakers for negotiating a better rate and spent $180,000 on two lobbying firms in 2011, according the National Journal.