Ethics is always a gray area on the Internet. The anonymity and the freedom to say almost anything, coupled with the fact that it is difficult to regulate what is being said and done on the cyberspace, contribute greatly to the blurring of what actions are deemed ethical and what actions are not.
It is usually up to the users of social media web sites if they wish to exercise restraint and good sense on their own actions or upon the actions of their fellow users. But if you have a company that is marketing its products, promoting its brand and finding its place on its niche market through its own social networking community, it would be disastrous for you simply to rely on the members of the community to use restraint and good sense. You are the one responsible for your own products, image and branding after all.