As Facebook gets ready to hit the stock market, Zynga’s place in its ecosystem appears to have changed since last year. The social network announced that the gaming company’s ads and transaction fees made up 15% of its revenue in the first quarter of 2012, compared to 19% in 2011.

In March, Zynga had launched its own gaming site at Zynga.com that would allow users to play with people outside their networks and to publish their scores without leaving the site. But Zynga continued to use Facebook Credits as its virtual goods payment system, which gives Facebook a 30% cut of the profits.

The payments, along with direct advertising from Zynga, generated 12% of Facebook’s revenue in 2011 compared to 11% in the first quarter of 2012. The additional 7% in 2011 and 4% in the first quarter of 2012 came from ads from other advertisers that Facebook displayed on Zynga’s apps.

“We may fail to maintain good relations with Zynga or Zynga may decide to reduce or cease its investments in games on the Facebook Platform,” wrote Facebook in an amended registration statement to the U.S. Securities and Exchange Commission. “If the use of Zynga games on our Platform declines for these or other reasons, our financial results may be adversely affected.”

Image by hjnisch via Shutterstock.