You fight with your employees: In some businesses, management and employees are constantly at odds. (An example was given of a unionized workforce where management-labor strife was common). This is also not the type of company that should encourage employees to communicate directly with customers via social media.
Management skepticism: If management doesn't believe in social media, then employees who have been told for years that public communication needs to be filtered will be hesitant to try out a new medium which requires them to speak openly. In this scenario, management needs to encourage and reward participation to make social media work. If they don't, it will fail.
Strategic Vacuum: Don't do social media just to do social media. If a company doesn't know what they're trying to accomplish, then there will be nothing to measure and no way to determine success. Just as with any other initiative a company takes on, there needs to be an objective...and that objective shouldn't be to distribute a press release.
Educators would be wise to examine if these are some of the reasons email, admin blogging and other forms of social media are failing in the public school systems.