Online game Moshi Monsters has 10 million registered users

"The free-to-play online children's game Moshi Monsters from UK-based developer Mind Candy has notched up more than ten million registered players.
According to the company's figures, more than one million new users are joining every month and the site has become one of the fastest growing children's sites in the world.
The social online game Moshi Monsters allows children to raise a virtual monster while solving educational puzzles that test their maths, spatial awareness, logic and vocabulary skills. Successfully completing puzzles rewards users with a virtual currency that can then be spent on customising their in-game home."
Source: GamesIndustry.biz, 1st December 2009