Have you tried calling them and just being honest?
"Hi, I wanted to resolve this before it became a matter of pride for both parties. We appreciate the service that you provided and this isn't a slight at your company. My dealership overvalued facebook’s potential as an asset of revenue on our balance sheet. The liabilities that we incurred from having a 3rd party monitor it full time have over shot our ROI and I am forced to make some changes. Can you bring me up to speed as far as your side of things, has your company been compensated for the set up of our facebook page?
[if yes]
"okay great, so moving forward what can we do to wrap this up? I just want this to get resolved and get our admin stuff back so that we can make sure it's compliant with Hyundai's dealer ad covenant and is up to date."
[if no]
"i had no idea, I'm sorry. what do I owe you? I just want this to get resolved and get our admin stuff back so that we can make sure it's compliant with Hyundai's dealer ad covenant and up is to date."
Sometimes it's best to ask for mercy and act like this is no big deal.