Disparaging clients, patients or products usually falls outside of the labor law’s protection, but choice of language is another matter. In 2010, the board sided with a woman who was fired from a Connecticut ambulance company after she went on Facebook to mock her supervisor by using a few vulgarities. “Love how the company allows a 17 to be a supervisor,” she wrote, using the company’s lingo for a psychiatric patient.

The NLRB said the woman’s comment drew supportive responses from her co-workers and led to further negative comments about the supervisor. The company, which settled the case before it went before an administrative judge, should have allowed the woman and her co-workers to exchange complaints about their managers, the NLRB lawyers said.

“This is a very new area, and employers are concerned about where it might go,” says Michael Eastman, executive director of labor law policy for the U.S. Chamber of Commerce. “We’re a long way from knowing where this will play out.”

Most of the cases Eastman reviewed in a comprehensive 2011 study involved nonunion workplaces. Employers were often surprised to learn that they were covered by this area of federal labor law, he says.

Sometimes, companies put in place comprehensive non-disparagement policies forbidding employees from disparaging management, fellow workers, the company, its products and its competitors, Eastman says. “The NLRB is taking the position [that] they can’t have a blanket policy.”

In the case of the BMW dealership, the judge found that parts of the employee handbook were overbroad and would tend to blunt employee rights. This included a “bad attitude” section that directed employees to display a positive attitude toward their jobs.

Richard Paul, a labor lawyer and adjunct professor at the University of San Diego School of Law, says that in addition to protected concerted activity, workers’ rights to use social media can be covered by other state and federal laws. In some states, he said, employees have the right to express political views without retribution at work. Other laws protect whistle-blowers or those who complain of safety violations.

“Beyond those four areas, employers are free to look at social media postings and measure the attitudes of the people who work for them,” Paul says. “Generally, they are free, at least in a nonunion environment, to make the judgment that they prefer to have employees who appreciate their workplace and who get along with management and their co-workers.”