That has the potential to change forever now though, as TechCrunch is reporting that Facebook has opened up a channel for any developer to run third-party advertising offers through its games, with those offers rewarding participants with each game's native currency. One example might be an offer in 6waves Lolapps' Ravenskye City, where a user can earn 4 Skye Credits (the game's native premium currency), just by signing up for a free account with Flixster (a movie discovery / rating network).
Since this isn't a mandatory feature, it might take some time for your favorite Facebook games to implement these offers, but once they do, you'll want to look for a large "Earn X Currency" button above your game, where X Currency is the name of that game's specific "cash." From there, you should spot a list that looks similar to the one below. Clicking on "Categories" will allow you to sort out only the free offers, like surveys and site registrations. Sure, you won't earn as much virtual currency for these offers as you would for, say, signing up for Netflix (a common offer currently seen across games), but it's all free - isn't that incentive enough?
Will you fill out free surveys or sign up for various social networks to earn free premium currency across your favorite Facebook games? Sound off in the comments!