It’s no secret that the primary source of income for social games is in-app purchases. As a model, it works just fine. Sometimes, though, other models work better for certain games – like the subscription model, for example. It’s coming to Facebook soon, and developer KIXEYE’s game Backyard Monsters will be one of the first titles to feature it.

There’s an element of irony here that’s impossible to avoid: MMOs are going free to play, and free to play games are about to start offering subscription services. It will be even more ironic if the subscription model takes off and developers prosper, but it’s a little too soon to draw any sort of conclusion like that yet.

The model is still in beta, with a full rollout expected to happen in July. When that day comes, all developers will be able to build their games around the model and enable users to earn certain benefits by paying a monthly fee. In the case of Backyard Monsters, $9.95 a month will net you things like in-app currency, customizable yard tiles, and more. If a quieter life is more your speed, Zynga’s Pioneer Trail and FarmVille will be testing the subscription model prior to the July roll-out as well.

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In addition to the introduction of subscriptions, Facebook will also be retiring its Facebook Credits and transitioning to the use of local currency. A Zynga spokesperson contacted Gamezebo today to let us know that their company is embracing these changes with open arms; “We believe Facebook’s new payment product to support pricing in local currency will streamline payment methods on their Platform and ultimately offer more flexible pricing options. And, while its early days, we are looking forward to be bringing players more options to engage in our games through our subscription model and have already begun testing subscriptions in a number of our games on Facebook.”

I imagine we’ll be seeing the model used in all sorts of different ways, ranging from unlimited play to rare loot. The possibilities are endless. Until then, we’ll all be able to poke around in Backyard Monsters and see just how well the model works.