I think it depends a lot on what you are offering and what a conversion is.
One thing I noticed from a few campaigns there, is that you will get a lot of 'curious clicks'. Remember people on facebook are generally not in any sort of buying stage and my experience has shown that sale conversions are harder to obtain from facebook vs AdWords or Yahoo PPC. Of course that is one opinion/experience and by no means the hard rule.
My recommendation would be to shoot for some type of opt-in from those ads vs trying to achieve a direct sale.