This is a risk we all take by using social media to promote our businesses. The biggest response that will hurt us is to respond in a knee jerk reflex and be defensive or to ignore it totally and appear as if you are hiding something. Your best response is to thank the critic for making you aware of the " problem" and if possible derail their negative comment by pointing out the positive aspects or measures that you/your company have taken regarding this even if it is something you already do as a company.