to gain credits you buy them using pay-pal but you can win some in some games such as bejeweled blitz. Also you gain 10 credits on your birthday but you can only gain it once a year so if you reset your birthday to collect another ten you will not receive it. I am sure there are many games to win credits but i only play bejeweled!
Games, applications and game prizes
You earn facebook credits by playing lots of games and apps on facebook.
Facebook Credits are a type of virtual currency that facebook users can use to buy virtual goods in their games and applications. As the program is still in beta (as of Feb 2010), only a select few applications use the system. A user receives 10 credits for every dollar that they spend, so the ratio from dollar to credit is $1:10.
No
You can buy Facebook credits with cash to help you excel in games and buy certain props.
If you redeemed your facebook credits then go to apps and games and it will say how many credits you have
Facebook itself does not produce games. However, Facebook hosts 'applications', many of which are games.
Games give you Facebook credits but you need to buy them.Actually some games do give you credits like car town and happy animals.
Search applications for them.
You can use your credits on Facebook games the most popular are Car town and Happy aquarium
mafia wars
You can get 1000 free credits when you suscribe on IMVU website and install it. Or you can try to get a chance with games on facebook.