Gaia Online, a San Jose-based virtual world and social networking site has managed to keep under the radar while attracting troves of teens - clocking two million unique visitors last month, with 85 percent living in the U.S., according to GigaOM.

With the tagline, "the world's fastest growing online world hangout for teens," the website is able to attract so many visitors through its reward program: the more time people spend in the virtual world, the more virtual gold they earn that they can use to buy items in virtual auctions.

The company's other revenue source is ad campaigns - that take the shape of immersive experiences created specially to run within the world of Gaia.

Gaia's campaign for New Line Cinema's fantasy adventure The Last Mimzy, for example, challenged users to accomplish a series of tasks in order to get their own special Gaian-only Mimzy, a super-intelligent bunny. Hundreds of thousands of these Mimzies were given out - meaning some 10-20 percent of their total user base jumped through the hoops to win the advertiser's prize. By contrast, when Nissan began giving away virtual versions of their cars in Second Life, fewer than 1 percent of residents took them up on the offer.