As was widely predicted, Facebook filed for its initial public offering today, with a staggering value of $5 billion. What wasn’t expected, though, was just how much of Facebook’s revenue came from the platform’s biggest game developer.

According to the company’s SEC statement, in 2011 Zynga accounted for 12 percent of Facebook revenue, making the FarmVille maker Facebook’s second biggest source of revenue. The largest is advertising, which accounted for 85 percent of Facebook revenue in 2011.

“To date, games from Zynga have generated the majority of our payments and other fees revenue,” the statement read.

In addition to providing revenue from virtual goods purchases, Zynga games also increase the number of pageviews on Facebook, which in turn leads to increased advertising revenue.