In 2006, facing low morale and high turnover, the Hills & Dales General Hospital in Cass City, Michigan, introduced a new code of conduct. It forbade gossip, encouraged teamwork, and included an employee pledge to "represent Hills & Dales in the community in a positive and professional manner in every opportunity."

But the code failed the Facebook test. This April, the National Labor Relations Board, or NLRB, ruled it illegal after a Hills & Dales employee was punished for defending a fired co-worker in a Facebook post that called the hospital's managers "douchebags." The NLRB found the "no gossip" tenet and its broadly worded "positive and professional" pledge-which Hills & Dales cited when disciplining that worker-violated staff members' rights to discuss work conditions without retaliation.

Social-media policy may not be an entrepreneur's primary concern, but in an age of tweeting My boss is a jerk, the NLRB is on a crusade, some attorneys say, to rewrite the rule book. Its cudgel: the nearly 80-year-old National Labor Relations Act, which lets workers discuss wages and working conditions. Generally, most companies with two or more employees must follow those guidelines. If you recently wrote a social-media policy designed to protect your business from rogue employees' tweets-or maintain staff morale by keeping workers' gripes private-you may have already broken the law.

A 2013 study from accounting firm Grant Thornton found that only about a third of all companies had social-media policies. "We dove into social media in 2010," says Mark Scovera, president of the Tallahassee, Florida, economic development firm Access Florida Finance. He made Facebook and Twitter its main marketing vehicles before it had any social-media policy: "We were flying by the seat of our pants." One concern, he later realized, was whether his social-media expert could bring Access Florida's Facebook and Twitter followers to a new job.

Also this April, the NLRB had Valero Services, one of the world's largest oil refineries, rescind its social-media policy. Other companies cited range from General Motors to Bettie Page Clothing, which sells apparel inspired by the bygone pinup star. Some workers fired for social-media misbehavior were reinstated and compensated for lost wages-a disruption to a company's workplace and finances that would make any entrepreneur wince.