Zynga has been experimenting with SMS in its petri dish of a social game, Mafia Wars, since late last year. But at that point it only let players get updates about what was happening in their games. It rolled out two-way SMS today, so players can actually make moves in the game from their phone.

The company likely thinks it can raise the average user’s engagement level by getting them playing through SMS instead of just on Facebook, and get them buying more virtual goods as a result. It may also be trying to find alternative means of regularly contacting users about game-play. Facebook removed third-party notifications at the beginning of March. Our AppData service doesn’t show the perenially popular mafia role-playing game taking any traffic dives since then, but perhaps the loss of notifications is affecting other factors we don’t measure, like average revenue per user.

Mafia Wars has hosted a number of feature tests that we’ve seen eventually rolled out to other games, like its browser toolbar. The fact that this particular app is text-based makes it especially suitable for SMS, although it’s not inconceivable that Zynga will find other uses for the medium, like letting people water their plants in FarmVille.

To start using SMS, go to the Mafia Wars app and click on the “Help” section (although some users may find it featured on the home page). To register, you enter your phone number, then Zynga send you a confirmation code and you enter that code back in the app. From that point on, you text MAFIA (623-42) and include a variety of commands. “FF” is “fight your last opponent again,” for example. You get a free virtual weapon if you sign up, with the promise of “daily bonus rewards” for continuing to play via SMS.