I buy services from Facebook, but recently I wasn't careful and exceeded my credit card's limit. Then Facebook sent me this:
Amount You Owe: $518.35 USD
We were unable to charge your ad account (43315396) using MasterCard *#### because you may have reached your card limit. Please add a new payment method or pay your card balance and try charging it again.
If I do not pay the bill (because I don't have the funds) then will the Facebook service get slower, or will they ban me?
Facebook may sue you or your company for not paying the bill for the ads that Facebook ran for you or your company. There is a chance they may ban you and/or your company in addition to that to get the money as there was likely an agreement you accepted to put ads on Facebook.
My guess would be that they sell your debt to a collection agency and don't really bother with pressing charges for 500 dollars. A collection agency can do multiple things to hold you accountable for your debt. I hate to bring it to you but it only makes sense that you should try to resolve this debt given that "not being careful enough" is not facebook's responsibility. I also assume they won't let you run any further ads on the same credit card.
What you could possibly do is call facebook and be honest with them, tell them it was an accident and that you can't pay the current amount. Maybe they'll waive the debt or give you a settlement offer.
I imagine the same results would occur as with any other business that is owed money. For a short period the company will try to collect their debts directly from the consumer. If unsuccessful, the company may then sell their right to the debt over to a collections agency. The collection agency will then pursue more aggressive collections tactics and/or legal action to collect.