Tapjoy has unlocked the Easter egg in the smartphone game: Its advertising platform shifted mobile gaming from pay-to-play to a freemium model. "We changed the industry," says CEO Mihir Shah. In exchange for trying ad-supported apps or interacting with an ad, users earn currency (to buy better weapons, for example). "We’re the largest performance advertising network," Shah says, noting that Android customers, for example, follow through on four ads a month. Tapjoy claims more than 8 million daily active users and is estimated to have generated $100 million in revenue last year, from advertisers such as Electronic Arts, Amazon, and Victoria’s Secret. The next level? Moving beyond games and into communications, news, and music apps.