Mattel's child-oriented 2.5D virtual world, BarbieGirls.com has reached ten million registered users (well more or less) according to the Washington Times. Long-time MMO/Virtual World observers and users will instantly note that a registered user does not an active user or subscriber make. However, Barbie Girls registered users require a special (customizable) Barbie MP3 player (called a Barbie Girl Device that range in price from US$40 to $US60) that has to be registered as a part of the signup process.
That makes things more interesting, and puts Barbie Girls as bigger than operations like Second Life which have similar ballpark numbers of registered users, but no such financial barrier to entry.
It's true that we don't normally write much about Barbie Girls - not because of any lack of merit as an MMO, no. We tend to avoid it because of the curse. They say that one of our writers once wrote a small news piece on Barbie Girls, and was inundated with messages about outfit unlocks and Barbie Bucks from millions of little girls. Forever. They say you can still hear him howling at GDC in San Francisco.