Facebook and Zynga do indeed have a special arrangement, according to a filing with the Securities and Exchange Commission.

Here it is, direct from a new filing with the SEC:

"The parties acknowledge that FB desires to enable Zynga to build the Zynga Platform on top of the Facebook Platform, and the parties desire to, amongst other goals set forth herein, work together to increase the number of users of each party’s products and services.

The parties further acknowledge that Zynga is making a significant commitment to the Facebook Platform (i.e., using Facebook as the exclusive Social Platform on the Zynga Properties and granting FB certain title exclusivities to Zynga games on the Facebook Platform).

In exchange for such commitment, the parties have committed to set certain growth targets for monthly unique users of Covered Zynga Games."

There you have it. This agreement appears to be the same agreement that appeared in Zynga's S-1 filing with the SEC.

The new filing, however, does disclose an ad revenue sharing agreement with Zynga. The clause is referring to Zynga's websites outside of Facebook.com that implement Facebook ads, according to another part of the filing:

Each month during the Term, for all Properties on which you implemented, during the previous month, the Facebook Ad Unit, we will pay you a percentage of Net Revenue (“Ad Share”) arising from such Properties for the previous month.

Here are some other tidbits from the filing:

Facebook "shall not offer or otherwise make available on the Facebook Site or the Facebook Platform any Facebook Game." Zynga can terminate its agreement if Facebook does.