FB Credits are bought with real world money, and used to purchase premium in-game items. Normally, games on Facebook already have their own kind of premium currency (e.g. Zynga's FarmVille Farm Cash), but some find a standardized Facebook currency to be more convenient for games developers and players.
There are already plenty of offers to earn game-specific premium currency, but now, Crowdstar has partnered with TrialPay to offer Facebook Credits to players. So theoretically, whatever FB Credits that players earn from these offers can be used elsewhere on Facebook, like the official Facebook store or rival games.
All you've gotta do is click on "Earn Credits" in any of your Crowdstar games, and follow the promotions offered. Sounds good, right?
For the promise of 15 FB Credits, I tried the "Ampario" survey, but after fussing with it for over 5 min. as it attempted to figure out whether I was eligible to participate, I was told that there were no surveys I was qualified for.
Oh well. While the concept is sound, these free deals tend be a huge hassle with zero pay-off. And not every Facebook games company pulls it off well. So far, the one that's done it best is Cafe World, but it's yet to offer FB Credits.