Apparently hockey fans like to draw: A tiny Draw Something competitor Doodle.ly hosted a similar promotion for the New Jersey Devils during the first round of the playoffs. So has the Minnesota Timberwolves basketball team, when it asked fans to doodle MVP Kevin Love.

Draw Something's new ad product came from testing recognizable brand terms like Nike , KFC and Doritos in the game, said former OMGPop CEO Dan Porter, now Zynga's VP-mobile and general manager for its New York office. "People loved to draw the Colonel and bags of Doritos," he said.

However, as brands begin to come onboard, Draw Something's popularity with consumers may be waning. Its monthly active users have declined in recent weeks, according to App Data. Daily active users have dropped from 14 million at the beginning of April to about 10 million.

Social-game giant Zynga bought Draw Something parent OMGPop for a reported $180 million in March. Zynga went public late last year and reported that 9% of its $311.2 million fourth-quarter revenue came from advertising.

Zynga makes the most of its revenue from small payments from gamers' buying extra perks. Draw Something, whose overnight success attracted Zynga and other suitors to OMGPop, has been known for integrating pop-culture terms, such as Lady Gaga, LeBron and characters from "The Hunger Games," into the game.

Since games are driving explosive app usage on phones and pricing in mobile-display ads races to the bottom, an ecosystem of not-just-banner-ad sellers has sprouted.

Startups like Kiip and Tapjoy are looking to get brands to underwrite rewards in games, for example, or gift a product for advancing through the game. Draw Something will soon integrate a similar service, said Mr. Porter, who declined to provide specifics.