I am sure, when presenting, I have told small business owners to not reply to negative comments online. If you do decide that you need to comment it would be along the lines of an apology. If there was a specific issue, you can reach out to them personally to either rectify the situation or let them know that the issue is or has been addressed.
You can't get hot and bothered and defensive. Just don't do it. The customer is right, even when they aren't, and if you try to defend yourself online you will just look stupid.
Need an example? Apparently Amy’s Baking Company Bakery Boutique & Bistro in Arizona were on Kitchen Nightmares with Gordon Ramsey. After two episodes, he walked out and said there was nothing he could do for them. Their response? Take it to Facebook. It is, quite honestly, one of the most horrible breakdowns I have seen by a business online. Crazy!
Go ahead. Read it. It is a "what not to do" in epic proportions.
What COULD they have done? NOT respond on Facebook. Really. The beauty of being a business owner is that you can choose not to play in the playground and instead focus on getting your stuff together. Really. These people, obviously, have some serious business and personal issues. They need all kinds of help, that they are not going to get by being defensive online.
What can YOU learn from this? Walk away. Don't sit at the computer and read the stupid stuff that people write about you. Start taking stock of your own business weaknesses and start addressing them in a professional and coherent manner. And please, for the love of all things, do not start flaming the fire online if someone DOES write something critical about you. Let it go. Write about it on an actual piece of paper, talk to a trusted friend, confide in a partner, but do not - do you hear me, DO NOT - flame the fuel by writing back, defending yourself, or getting into it with online peeps. You are ALWAYS better than that.
You can see them in this episode of Kitchen Nightmares: