Flying under the proverbial radar for the last four years, the web-based virtual world chatroom IMVU has released new jaw-breaking data: Since April 2004, it has amassed 20 million registered accounts, with 600,000 of those active monthly users. By comparison, Second Life took five years to acquire about 550,000 active users.

The company, well known to web surfers because of its ubiquitous ads, is now earning $1 million a month in revenue, 90 percent of that from the sale of virtual currency and 10 percent from banner ads embedded in its interface, CEO Cary Rosenzweig said. That works out to about $1.66 a month per active user. By VC Jeremy Liew’s estimate, market leaders Habbo Hotel and Club Penguin are earning $1.30 and $1.62 monthly average revenue per user, respectively. How did IMVU grow?

Most of IMVU’s massive catalog of avatar clothing, accessories and other objects available on its online catalog are made by the users themselves. They purchase those items from each other with IMVU credits. (A block of 1000 costs you $1.) IMVU then takes a cut of the profits for each virtual item sale, with the rest going to the individual user.

The result: fresh quality content produced on a regular basis by energized creators. “It’s my personal belief there’s maybe in the order of dozens who are doing this for a living,” Rosenzweig said. “Perhaps hundreds who are doing it for spending money.”

Some more notes:

• About 1.7 million user-generated items have been uploaded to the IMVU catalog
• Users create most content offline with industry-standard modeling tools such as Maya and Blender
• Individual content creators can take their earnings and sell them on third party web sites
• As with Second Life, IMVU users retain intellectual property rights to content submitted to the IMVU catalog
• About 100,000 users are registered to be IMVU content creators, but the number of consistent creators is in the tens of thousands
• IMVU users often sell their credits to other users via third-party web sites — also fine with Cary. “We observe it sort of casually,” he said.
• Demographic: Largest cohort is teens; 60 percent of users are female, 60 percent are American
• 50-70K concurrency is typical, with peaks in the high 70s
• IMVU areas are more like virtual rooms restricted to 10 avatars or fewer; Cary estimated 90 percent of those room interactions are person-to-person, anyway.

While not yet profitable, the company plans to roll out pre-paid IMVU currency cards in Target, Blockbuster and other major retail chains in the next two weeks. With light 3-D graphics and cute-sexy cartoon avatars that appeal to girls and young women, it’s easy to see IMVU dominating its particular niche in the virtual world ecosystem, roughly dead center between Second Life and Barbie Girls.