And, so the industry legend goes, the "virtual good" was born. In the years since, digitally created items for games and social media - from beers on Facebook to weapons in role-playing worlds - have grown into a $15 billion globe-spanning business. They helped generate the likes of Zynga Inc and underpin a fast-expanding online-gaming industry.

Now, companies such as Nexon that pioneered the model across Asia are muscling onto Zynga's turf, hoping to cash in on fast-growing U.S. spending by bringing their distinct Asian brand of fast-paced "freemium" games - free with optional in-game purchases.

With PC sales stagnating even in China, the industry's heavyweights are increasingly doubling down on the mobile space and targeting the Western market as more smartphones and tablets land in users' hands. Encouraged also by slowing growth in casual games on Facebook - basic time-killers where game play can be measured in minutes rather than hours - they have stepped up their investments this year.

"The emphasis of the Asian publishers is to successfully set foot in Western markets, and that gives you a blueprint of what's to come," said Joost van Dreunen, managing director of digital goods intelligence firm SuperData Research. "The free-to-play games for mobile are where we identify huge growth."

Asian gaming bigwigs began exploring the Western market a few years ago with high-engagement, addictive and often hard-core Web-browser titles. But this year has brought a spike in activity that threatens to undermine Zynga's own efforts to diversify into mobile gaming and wean itself off Facebook Inc.

Japanese social gaming powerhouse DeNA Co aims to enlarge its social and mobile gaming network Mobage in America. It is releasing games based on the Marvel superhero and Transformers franchises. GREE Inc has bought two San Francisco-based mobile game developers since May and is spending heavily to acquire American users.

Nexon, which went public in December, also wants to be a prominent American player and is gearing up for a massive launch of "Epic of the Three Kingdoms" in 3D for PC and mobile devices.

The new competition comes at a bad time for Zynga, the $2 billion company behind "FarmVille" that gets over nine-tenths of its revenue selling virtual goods: maps, weapons and other items that give players an edge that are offered for small amounts, often under $1, as enhancements in games that are free to download and play.

The company helped convert large numbers of nongamers into players through colorful, less time-consuming casual games like "CityVille," but is now struggling to monetize mobile games, stanch executive losses, and resuscitate a floundering stock that has dropped 70 percent since the beginning of this year.

"Zynga and other American game companies are already feeling some pain from the marketing and advertising side from GREE in particular, but also DeNA, in terms of the significant amount that they are spending on acquiring users in the U.S.," said Colin Sebastian, an analyst at Robert W. Baird.

NOT A FAD

The corporate logo of Zynga Inc, the social network game development company, is shown at its headquarters in San Francisco, California April 26, 2012. - Reuters Photo

Asia's gaming titans hope the United States will be the source of the industry's next phase of expansion, perhaps at Zynga's expense, in a reversal of how Western companies typically rely on emerging markets to bolster growth.

With the proliferation of mobile devices the worldwide market for virtual goods will surpass $20 billion by 2015, according to SuperData Research. U.S. gamers spend much less than Asians, but their expenditure is growing faster.

Revenue from U.S. virtual goods is expected to grow 36 percent to $3 billion this year and double to about $6 billion in two years, versus shrinking sales of traditional video game products, according to Bonnie Ho, lead research analyst at Inside Network.