Facebook is better used as a customer-loyalty program, not a customer acquisition program. Some interesting facts:
What does this mean? Facebook needs to be an integral part of your online marketing plan, targeted at maintaining loyalty with your fans. Stay in touch with the people you know, get their feedback, reward them, but don't plan on Facebook getting you lots of new customers. Some, yes, but it's more useful at keeping the ones you have and increasing their loyalty.
http://adage.com/article/digital/brands-facebook-a-loyalty-program/229561/