I was just surfing through net when I found lots of people having confusion about how to convert fb credits into farm cash.
Now almost all users of facebook must have got some free fb credits so converting them into farm cash is like getting free farm cash.
Now 10 fb credits = 1 dollar and in 1 dollar you can get 4 farm cash.
Here are the steps describing how you can convert your FB credits to farmcash.
1. Open farmville in facebook and not in farmville.com, because it gives lot of problem to convert there(I wasn't able to do it there :) ).
2. After opening your farm click on the add farm cash or coin button as indicated by arrow in the picture below.
If you have another 10 fb credits you will get another 4 farm cash on repeating the process.
I hope this information was helpful.
Yours,
Smart Farmer.