Both Zynga and Facebook have been going through some tough times lately, and things aren't getting any better, as it has been revealed that Zynga is now generating a smaller percentage of Facebook's revenue than it did previously. According to a new Facebook SEC filing, Zynga accounted for an estimated 14 percent of the social network's revenue during the first half of 2012, down from 19 percent during all of 2011.

After its IPO announcement Facebook had revealed that Zynga was responsible for 12 percent of its revenue in 2011, but the new 19 percent figure factors in more revenue streams, hence its larger size. In addition to virtual goods sales and direct advertising — which accounted for the original 12 percent — the new figures include "revenue from third parties for ads shown on pages generated by Zynga apps, and Facebook ads and Sponsored Stories displayed on Zynga.com."

"If Zynga does not maintain its level of engagement with our users... our financial results could be harmed."

Zynga may be shifting its focus from the social network to mobile gaming, but Facebook recognizes the importance of the FarmVille developer to its bottom line. "If Zynga does not maintain its level of engagement with our users or if we are unable to successfully maintain our relationship with Zynga," the new SEC filing reads, "our financial results could be harmed."