This afternoon “The Facebook Effect” author David Kirkpatrick, and Napster co-founder and former Facebook president Sean Parker, joined TechCrunch’s Michael Arrington on stage at the TechCrunch Disrupt conference to discuss the book, and Facebook in general.

The discussion hit many topics, but one of the more interesting tidbits came late in the session in response to an audience question about Facebook’s next big revenue source. In response, Parker said that he thinks Facebook’s new Credits system, which the company will soon push as its payment platform, will account for one-third of its income in the next 12 months.

Given that Facebook will be earning a 30 percent cut from transactions using Credits and that it’s the only payment platform the social network is allowing developers to use, that prediction doesn’t seem too unbelievable. Social games company Zynga was one of the biggest developers to fight back against the Credits system, but it recently relaxed its position and entered into a five-year contract with Facebook.

Facebook has been positioning Credits as an easy way for users to buy virtual goods across various games on the Facebook platform. But with over 10,000 games available, and over 200 million users who play a Facebook game at least once a month, you don’t have to be an expert on the site to see just how easily Credits could quickly become an important source of revenue.