I'd say it's not sustainable since there is a limited lifetime value of a user in their game space. In order to continue increasing revenue you must grow new users at a rate that exceeds the loss of existing or prior users that have aged out of that phase of "spend money in game."

I suspect that Zynga IPO'd near the top of their possible revenue and without the ability to constantly acquire new users and/or re-engage previous users who have moved past spending money in-game they are going to decline over time. Even if they establish a stasis between loss of user revenue vs new revenue their share price will have to drop since the P/E multiplier would be that of a matured company rather than a growth stock. Share price would be discounted accordingly.