In its third letter offering guidance to employers on social media, the National Labor Relations Board said employees cannot be prohibited from discussing their jobs on Facebook or Twitter and should not be told not to friend coworkers.

The memorandum, issued by Acting General Counsel of the NLRB Lafe Solomon, focused on recent cases challenging the social media policies of seven companies seeking to regulate employee usage of sites like Facebook and Twitter. With topics ranging from intellectual property to privacy, the agency found portions of six of the companies’ social media policies unlawful. It upheld the entire policy of just one company.

Examining General Motors’ social media policy, the NLRB said that a provision “instructing employees to ‘Think carefully about “friending” co-workers’ is unlawfully overbroad because it would discourage communications among co-workers.”

Further, the carmaker’s rule requiring an employee to receive permission prior to posting photos, music, videos, quotes, or personal information – including employer logos and trademarks – is also unlawful, the NLRB said. Without further explanation, employees might believe that such limitations could include photos of picket signs containing the company’s logo or employees working in unsafe conditions, which would constitute protected activity. “Although the employer has a proprietary interest in its trademarks, including its logo if trademarked, we found that employees’ non-commercial use of the employer’s logo or trademarks while engaging in [protected activity] would not infringe on that interest,” Solomon wrote.

In a second case, McKesson Corp. cautioned employees in its policy about the use of copyrighted material, urging them to “respect all copyright and other intellectual property laws.” While the NLRB determined that such an admonition was lawful, a portion of the rule requiring prior permission before reusing content and images from other parties was unlawful.

Clearwater Paper Corp.’s policy raised problems when it urged employees to “avoid harming the image and integrity of the company.” The provision is unlawfully overbroad, as employees could reasonably construe it to prohibit protected criticism of the company’s labor policies or its treatment of employees, the agency said.

And Dish Network ran afoul of labor and employment laws in its policy, which prohibits communication with the media and members of the press, including blogs, forums, and message boards, or speaking at a conference or seminar. “Employees have a protected right to seek help from third parties regarding their working conditions. This would include going to the press, blogging, speaking at a union rally, etc.,” Solomon wrote.

In contrast, Walmart’s social media policy was found to be completely lawful, the agency said. A section titled “Be Respectful” could have been overly broad with its suggestions to be “fair and courteous” when posting comments, complaints, photographs, or videos, the memo noted. But Walmart’s policy “provides sufficient examples of plainly egregious conduct that employees would not reasonably construe to rule to prohibit [legitimate] conduct,” like tips to avoid posts that “could be viewed as malicious, obscene, threatening, or intimidating,” the NLRB said.

To read the memo from the NLRB, click here.

Why it matters: Companies updating or drafting social media policies can look to Walmart’s policy for guidance, particularly the company’s use of examples. “Rules that clarify and restrict their scope by including examples of clearly illegal or unprotected conduct, such that they could not reasonably be construed to cover protected activity, are not unlawful,” the NLRB said.