Playdom, the social gaming startup sold to Disney earlier this week, has signed a five-year contract for the exclusive use of Facebook Credits, Facebook's virtual currency, in all Playdom games.

Demonstrating clearly that Credits matter greatly to the social networking service going forward, Facebook has of late focused on fostering deep relationships with its most popular third-party developers, often ensuring that Facebook Credits play a major role in their services. The revenue could be important for the social networking site, since Facebook takes a 30% cut from all Credits income.

As early as May, social gaming startup Zynga entered a five-year relationship with Facebook that somehow involved the expansion of Facebook Credits in Zynga's games. While today's news says Playdom has committed exclusively to the virtual currency, Zynga may have had a little more leverage to use against Facebook in forging their deal. That is, Zynga did not have had to commit to just one system.

Crowdstar was the first to commit exclusively, however, when in late June the company promised to use only Facebook Credits for a full five years. Others, like LOLapps, RockYou, Wooga, and some smaller app developers, have also decided to only use Facebook's own virtual currency.

Earlier this week, Disney announced that it had agreed to pay $563.2 million up-front for Playdom, the largest social gaming publisher on MySpace and the third-largest on Facebook, continuing the entertainment company's aggressive push into mobile and social gaming.

With today's announcement, we can safely say that Facebook Credits have the backing of Disney itself, a huge win for Facebook in its efforts to make Credits a widely used world-class virtual currency platform.