Reply immediately. Letting negative comments or reviews go unanswered shows that you and your business' employees do not care about a customer's concerns.

However, it is best to take a negative conversation offline as soon as possible.

Post a simple message and request the customer to contact your support team directly. This way, whatever followup that takes place is out of the public eye.

Here's an example:
 "I'm sorry your experience did not match yours and our expectations. Please send us a direct message or email/call [insert contact info] so we can further help you."

Then you can follow up with any remedial offer that is necessary.