Most employers who operate workplaces without labor unions think that they are unaffected by the pronouncements of the National Labor Relations Board. They should think again. In a series of recent cases, the NLRB’s general counsel has taken the position that employee activity on social-media sites like Facebook and Twitter can trigger certain rights under federal labor law for even nonunion employees.

For example, in May, the NLRB issued a complaint alleging that a BMW dealership outside Chicago violated the rights of a nonunion car salesman who used his Facebook page to complain about the quality of the food his employer had served at a sales event, complete with photos of the offensive hot dogs and water bottles. The dealership fired him for his posting, which was accessible to his fellow salesmen.