London-based developer Mind Candy announced that Moshi Monsters, a free-to-play social online game allowing kids to adopt and raise pet monsters, reached 10 million registered players.

Launched in April 2008, the browser-based MMO has players choosing from one of six monsters to customize and nurture, decorating their homes, playing minigames (e.g. a Bejeweled-style puzzer), and solving puzzles that test skills like basic math, spatial awareness, logic, and vocabulary.

After completing puzzles, kids earn Rox, the in-game currency used to purchase accessories, pet food, and furniture for their homes. Though Moshi Monsters is free to play, the game offers a $5.95 monthly subscription promising access to exclusive areas, features, and a Rox tree players can shake for free money.

Mind Candy says more than one million new players join the Moshi Monsters site every month. Its players have created more than 10 million monsters to date, played over 120 million educational puzzles. They also spend a lot of Rox, purchasing more than 1 million virtual items each day.

"We’ve been astounded with the growth of the game this year," says Mind Candy's CEO Michael Acton Smith. "Most of the growth has been due to word-of-mouth which is a great endorsement from our players that they love the Moshi world and want to share it with friends."