85% of revenue is from advertising. So either Zynga controls 80% of all non advertising revenue (aka the remaining 15%) or that 12% figure includes advertisements served on Zynga pages.
Is it reasonable to conclude that Zynga constitutes 80% of FB's credit ecosystem?
No, the 12% includes advertisements Zynga buys from Facebook, but not ads that appear on their pages.
"In 2011, Zynga accounted for approximately 12% of our revenue, which amount was comprised of revenue derived from payments processing fees related to Zynga’s sales of virtual goods and from direct advertising purchased by Zynga. Additionally, Zynga’s apps generate a significant number of pages on which we display ads from other advertisers."
Sadly the answer is yes (it is reasonable to assume 80% of FB's credit ecosystem targets Zynga assets). Check out Zynga's S-1 (since, thanks to the deal, most of zynga's revenue comes through FB credits, and those numbers will have been discounted because of FB's cut)
Facebook does not take a cut of third-party display advertising.
For example, if an advertiser pays Zynga to put a banner ad in their application or above the Flash widget containing their game, Facebook doesn't see a penny of that.
Zynga pays Facebook to advertiser in the right-hand column, though. Lots.
Additionally (and this doesn't count towards advertising revenue), Facebook Credits are mandatory for all games on the Facebook Platform, and Facebook takes a 30% cut any time a player purchases some.
Are those ads counted toward the 12% statistic?