Last week, RockYou began telling users (and us) about its plans to make Credits the exclusive payment option in its Facebook applications. Today, the company has gone further, announcing a five-year deal to exclusively use Facebook’s in-house virtual currency on the platform.

RockYou is one of the largest developers on the platform, with around 34.6 million monthly active users and 2.70 million daily active users, according to AppData. By getting this commitment, Facebook is ensuring that the company’s user base will start using the currency, thereby furthering its reach across the platform.

Facebook intends for Credits to get more users buying more virtual goods on third party applications than they have through third party payment options. It is experimenting with a variety of promotions to help spur the currency along, including giveaways, and special advertising for third-party developers who adopt it — also signing up other developers to exclusive five-year deals, including CrowdStar and Lolapps.

Meanwhile, some developers have balked at Credits, because Facebook takes a 30% cut — whereas it has taken 0% up until now — and because the implementation of Credits can create additional costs for developers.

For more, see our coverage from last week about RockYou’s Credits integration.