19 Months Ago, John Rice from edugamesblog (WordPress) says (in Virtual Worlds for Children Survive and Thrive) The subscription volume is amazing. Habbo: 90 million subscribers. Neopets: 45 million. Club Penguin and Star Doll: 15 million each.

For Mr Seiler [editor of Virtual World News] this popularity is based on three factors: friends, freedom and fun.

References:

And Izzy Neis from izzyneis (WordPress) says (in BCC's Booming youth VWs) While some of that total are recognisable games, most should be classified as virtual worlds, said Joey Seiler, editor of Virtual Worlds News, who drew up the exhaustive list.

Although these worlds, such as Club Penguin and Neopets, have games in them they are more an environment childre...

20 Months Ago, Animation World Network says (in New Nickelodeon Worlds in Development: Monkey World and SpongeBob ... - Animation World Network) New Nickelodeon Worlds in Development: Monkey World and SpongeBob ...
 CA - Apr 4, 2008
There will also be increase in interactivity on its Neopets site via a new virtual world called World of Neopia. A paid tier to Nicktropolis is also ...

And National Post says (in Nickelodeon explores virtual worlds - Canada.com) Canada.com
Nickelodeon explores virtual worlds
Canada.com, Canada - Apr 4, 2008
The network also will announce that it will increase the interactivity on its Neopets site through a new virtual world called World of Neopia, ...

And WebProNews says (in Nickelodeon Adding Virtual Worlds - WebProNews) Nickelodeon Adding Virtual Worlds
 KY - Apr 3, 2008
Nickelodeon is also creating a virtual world around its Neopets site called World of Neopia, and will add a paid option to Nicktropolis, the virtual world ...

And VFXWorld says (in SpongeBob, Monkeys Head Online For Nickelodeon, VooZoo Expands - VFXWorld) SpongeBob, Monkeys Head Online For Nickelodeon, VooZoo Expands
 CA - Apr 3, 2008
They will also announce an increase in interactivity on its Neopets site via a new virtual world called World of Neopia. A paid tier to Nicktropolis is also ...