Sony’s Free Realms has scored 3 million registered users in just seven weeks, making it a true hit for the free online games business model in the U.S.

The “free to play” games, where players start playing for free but can purchase virtual goods with real money as they add new capabilities, has taken off in Asia, where companies such as Shanda and Nexon have pioneered this new way of doing business. But until now U.S. consumers have been reluctant to embrace it.

Sony Online Entertainment’s Free Realms is the first big stake in the ground for a free-to-play massively multiplayer online game in the U.S. market. It could force a lot of rivals to follow and, god forbid, might even make a dent in the demand for the industry-leading World of Warcraft, which has 13 million paying subscribers. Just two weeks ago, Sony announced it had two million registered users for Free Realms.

Free Realms launched on April 28 with a focus on younger players who enjoy fantasy role-playing with a more cartoonish art style. John Smedley, president of Sony Online Entertainment, said at our recent GamesBeat 09 conference that Free Realms was aimed to get kids playing with their parents.

The game is rated E10+ for anyone ages 10 and up. Sony sells game currency cards for Free Realms in five major retail chains in the hopes of snaring young players who can’t use credit cards to pay for items they might purchase online.

He said the virtual world is adding an average of 500,000 players per week. Sony is granting a party pack, or virtual goods that the players can use with their virtual pets. Players can choose between 20 different types of cats and dogs to train, feed, groom and dress up in the game.