Just in via Twitter: Pandora has registered its 20 millionth user. The three-year old music streaming service is trying to hold its own despite tough economics and recent layoffs.

Its music-recommendation engine pumps out personalized radio stations over the Web, and its iPhone app remains one the top free apps on iTunes (currently No. 21).

How many of those 20 million registered users are active? Most of its listeners are in the U.S. According to comScore, Pandora had 5 million unique U.S. visitors in November, more than CBS-owned Last.fm (2.8 million) and close to Imeem (5.5 million). (MySpace Music, in contrast, had 17. 8 million). Here is a Google Trends chart showing a similar relationship.

Update: According to the company, 40 percent of its registered users, or 8 million, have been active in the past 90 days.