With over 100 million players, online gaming has advertisers clamoring to get a piece of the action. Dollars spent advertising with online casual games increased roughly 20 percent last year, to about $150 million, Ad Age writes.

Much of that increase is due to a shift in how casual games are being offered. In the past, they were offered online in as a free trial, with the option to upgrade to a premium version with more features, for a fee. Now games companies are offering gameplay for free, if the users agree to watch ads.

"Only 2 percent of people buy the game they try, but you can sell the other 98 percent by advertising on a per-play system, and it works very well," said Alex St. John, cofounder and CEO of WildTangent.

"Our biggest challenge is inventory. We're completely sold out through the next quarter, so we're out signing up more games for the network."