Clear Channel’s iHeartRadio has surpassed 30 million registered users less than two years after the launch of its current version. The milestone puts iHeartRadio at roughly 43% of Pandora’s 70.5 million registered users in the United States, Australia and New Zealand (although the service fully launched in the latter two countries in December).

iHeartRadio may have narrowed the registered users gap, but its users don’t listen as much as do Pandora users. According to Triton Digital, in March iHeartRadio had only 17% as many average active sessions and 15% as many total session starts as did Pandora. If iHeartRadio’s 30 million registered users listened as frequently as Pandora users, it would have had 64% more active sessions and 56% as many total session starts.

Clear Channel launched the current version of iHeartRadio in September 2011. The service offers online streams of radio broadcasts as well as a personalized listening experience a la Pandora. Over time Clear Channel has added the broadcasts of Cox Media Group, Emmis Communications and college radio stations.

That iHeartRadio has amassed 30 million registered users in a relatively short period of time — it had 20 million registered users just seven months ago — speaks volumes about the state of Internet radio (and Clear Channel’s efforts to market the service during broadcasts and through its annual iHeartRadio music festival). Listeners are easily adapting to new services and integrating online radio into the other ways they experience music. There is enough upside in Internet radio to allow for numerous services to grow. Pandora has continued to add registered users and has experienced a decline in listener hours only because of limitations placed on mobile listening. Songza has surpassed 6 million iOS installs, up from 2 million at the end of the summer, and had 4.7 million active users in March. [Billboard.biz]