Since going "live" two months ago, Mattel's online world Barbie Girls has signed up 3 million members and continues to add 50,000 new members a day, reports GigaOM.

It took subscription-based World of Warcraft four months to reach just half that amount, sparking GigaOM to bill the subscription-free virtual world as "World of Barbie-Craft."

Barbie Girls is one of the many examples of how virtual worlds are being dominated by pre-adults. About half of the sites on GigaOM's Top Ten MMO list are kid-oriented: Habbo Hotel, Club Penquin, Webkinz, Gaia Online, and Runescape.

Other kid-oriented sites that are popular, but not on the list, are Zwinky and NeoPets.

Unlike World of Warcraft, almost all of these are subscription-free, relying on advertising/sponsorship deals and sales of virtual items.