If it's a company controlled fb page, then yes, people should be free to go on and say they don't like the product for whatever reason. And it would be wise if the company read both negative and positive comments. If it is a fb page set up more as a "fan page" then having a conversation that debates pro/con can also be helpful, if done in a polite way. No product is perfect, and if a site is limited to just uber-gushing about a product, I'd find it very suspicious and probably just a bunch of propaganda. But yes, some people are bored and do like to let their complaints be heard.