It used to be that word of mouth was just that. One person talking to another. And in most cases the people knew each other. They were friends or neighbors or relatives who had some sense of who the person they were taking advice from was.
But the Internet, with its plethora of blogs and social sites, has brought about people communicating in a completely anonymous virtual environment in which they have no real idea who the other person is. Furthermore, there is no real accountability. Anyone can post a comment or a "tweet" about anyone else with no oversight as to whether or not the statements they make are true or not.
We had a disgruntled employee post a completely untrue and actually slanderous statement about the way we ran our business that resulted in our having to spend a considerable amount of time attempting to ameliorate the potential damage such statements could cause. The most frustrating part of this was that the employee posted these statements anonymously using a pseudonym so that she could not be identified. We knew it was her but because she chose to hide behind a clever alias, we never had a chance to confront the issues directly.
What makes this even more difficult is the fact that with many trusted sites, like Wiki for example, anyone can edit the pages at will. So you can never know who is writing what you are reading or if the statements they make have even a modicum of truth in them. Word of mouth has never been more untrustworthy. One of the downsides of the Internet.