Many organisations don't want start a blog or Facebook fanpage because they're afraid of people contributing nasty or unwelcome comments.
But, with a little fore thought and advance planning, it doesn't have to be that way.
the number one rule when responding to all criticism, even the negative type, is to stay positive. Sound defensive or negative and you'll fuel the fire. Adding more negativity to the conversation by letting yourself be drawn into a fight with a customer or user will only reflect poorly on your orgs.