How do I contact Facebook about potential fraud with my account?

I have gotten an email bill from Facebook and I don't know why since I did not sign up for "Ad Manager" (Which seems to be a system so you can put your advertisements on Facebook. But I do not arrange to use this feature).

I think  that maybe Facebook or someone else made a mistake or  worse, someone intentionally  hacked my account and  is having the  bill sent to me.

As far as I know, no money is being taken out of my  bank account since I have not signed up my bank account with Facebook. But what could happen is if I don't pay the bill (why should I?)  then Facebook might take action against me for not paying?  What should I do?  Just tell them that  someone is somehow using my account to have them send bills to me? How do I  reach Facebook? There is  no  link or  information on Facebook as to what to do in case of  account fraud like this?  

Depending on the answers I will provide more information or ask more questions