The National Labor Relations Board (NLRB) has issued a number of rulings that protect employees’ right to express their opinions on Social Media. The New York Times reported that NLRB Chair Mark Pearce noted that "federal law has long protected the right of employees to discuss work-related matters":

Many view social media as the new water cooler,…All we’re doing is applying traditional rules to a new technology.

The NLRB relies on the 1935 National Labor Relations Act which protects "the rights of employees and employers, to encourage collective bargaining, and to curtail certain private sector labor and management practices, which can harm the general welfare of workers, businesses and the U.S. economy."

NLRB rulings:

…apply to virtually all private sector employers, generally tell companies that it is illegal to adopt broad social media policies — like bans on "disrespectful" comments or posts that criticize the employer — if those policies discourage workers from exercising their right to communicate with one another with the aim of improving wages, benefits or working conditions. 

Recently NLRB ruling have led major companies to rewrite their Social Media policies including General Motor, Target, and Costco. It will be interesting to see how Social Media policies are interpreted by courts in the future given the NLRB rulings.