Like many businesses, retailer Costco Wholesale Club had a social-media policy that prohibited employees from posting statements online that could potentially harm the company's reputation or damage any co-workers. Those restrictions might seem fair enough, from a business owner's point of view. However, the National Labor Relations Board ruled in September that such bans are illegal because they could prevent workers from engaging in "concerted activity," or speaking with each other online about salaries or working conditions. "People are really surprised to hear that employees—even though they're not in a union—have a right to engage in concerted activity," said Beth Milito, senior executive counsel of the National Federation of Independent Business Small Business Legal Center. In a media capital such as New York, where many employees are active on sites like Facebook and Twitter, the ruling is worth noting. All U.S. businesses engaged in interstate commerce are subject to the NLRB's jurisdiction, even companies that aren't unionized. The NLRB, an independent federal agency charged with protecting employees' rights, typically considers companies with revenue of at least $500,000 to be engaged in interstate commerce, but in some cases businesses with even less revenue fall within the agency's jurisdiction. Costco–which did not respond to a request for comment on the ruling–is hardly the only company to run afoul of the NLRB. Administrative law judges for the agency have issued several decisions in the past year criticizing social-media policies that restrict workers' ability to talk with each other online. Officials also have said that in some cases, firing people for complaining online is illegal. The stakes for businesses that make mistakes on this front can be quite high, given that the NLRB can order companies to rehire people who were wrongfully terminated—with back pay. Even firms that are merely accused of having problematic policies (as opposed to wrongly firing employees) face potentially large litigation costs if they end up having to defend their policies to the agency. So how should entrepreneurs deal with employees' commentary on Facebook or Twitter? Here are some tips from attorneys.