Virtual goods have been popular in Asia for years. In the United States though, only ardent video game fans spent money on them, mostly for swords and spells in virtual fantasy realms. That is rapidly changing, driven by the popularity of widely appealing games for social networks like Facebook and mobile phones like the iPhone.

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"It's an experience, like going to the movies. That's how I describe it," said Sara Merrill of Parsonfield, Me., who plays Pet Society on Facebook with her two young sons five times a week. Recently, the family used a credit card to buy $20 worth of the game's currency, then bought items like a haunted mirror and a potion that helped their pet, Demon Baby, grow bat wings. "It's still cheaper than taking the kids to Target where they will ask for a toy," she said.

For outsiders, the selling of virtual goods -- items with no actual value in the real world -- might seem the very definition of a swindle. But often, strong -- and somewhat rational -- motives are at work. Users of social networks can buy one another gifts, like images of flowers and birthday cakes, typically for a dollar each. Facebook recently expanded its gift store to allow other companies to list their virtual wares, like greeting cards.

"It's not about the good itself, it's about the underlying human emotion or desire," said Moshe Koyfman, a principal at Spark Capital, which has invested in two virtual-goods start-ups. "The recipient knows the person took time, picked something meaningful and spent money on it."