A long awaited partnership between Yahoo and Zynga is finally coming to fruition this morning, as Yahoo announces an expansion of its gaming portal and other sites. Zynga will place two games, Mafia Wars and FishVille, on a set of Yahoo properties including Games, Messenger, My Yahoo and the Toolbar.

Yahoo’s announcement comes just a day after Microsoft announced its own deal with CrowdStar, and signals that the big web portals are still highly interested in social games. Both big companies are integrating games across multiple properties, connected by both Facebook Connect and their own APIs.

Zynga has its own reasons to be interested in expanding beyond Facebook. Also yesterday, Zynga CEO Mark Pincus mentioned at the ongoing Web 2.0 Summit that Zynga has served some 320 million players. Almost all of those will be from Facebook.

Having touched most of the available population of gamers on Facebook, Zynga needs to find and foster new channels. The catch is that this process is painstakingly slow, when compared to the speed with which Zynga started on Facebook: the Zynga / Yahoo deal was first announced in May.

Even with the deal done, there’s no guarantee that it will satisfy Zynga. Before CrowdStar joined forces with Microsoft, Zynga had its own deal with that company. But in August, Zynga pulled FarmVille off MSN Games, seemingly dissatisfied with the results.