WASHINGTON -- In an effort to control employees' activities on Facebook and Twitter, some U.S. companies have instituted social media policies that run afoul of labor law and infringe on workers' rights, according to a memo issued Wednesday by the general counsel of the National Labor Relations Board.

General Motors, one of the largest automakers in the world, has gone so far as to forbid employees from posting anything potentially "misleading" about the company online and even told employees to be careful about "friending" their co-workers on Facebook, the memo indicates.

Other companies, including DISH Network and Target, have also maintained corporate policies that at least partially violated provisions of the National Labor Relations Act, according to the memo. That federal law covers collective bargaining and employees' "protected activities" in the workplace.

Written by Lafe Solomon, acting general counsel for the NLRB, the memo is meant to help employers navigate the ever-changing territory of social media and develop workplace policies that keep within the bounds of the law. Solomon essentially acts as the lead prosecutor for the labor board, and the memo lays out his conclusions on a number of issues that have recently come before the agency.

"This [memo] is actually in response to requests from employer groups who said, 'Hey, we need some guidance. What's a good social media policy?'" explained NLRB spokeswoman Nancy Cleeland. "Social media just by its nature is collective activity, and that goes to the heart of our law. It makes sense that we would pick this up."

At this point, Solomon's reading of the law constitutes little more than suggestions to employers on how to avoid potential legal problems. And a GM spokesperson said that despite the NLRB's recommendations, the company hasn't changed its policy, maintaining that it "complies with applicable laws." But the memo may signal some of the legal wrangling that lies ahead, particularly if more workers choose to challenge their employers' social media policies on the grounds of free speech or labor rights.

The memo quotes social media guidelines from several companies whose policies came up in cases before the board and then provides an analysis of their legality. Judging from those guidelines, companies are trying to walk a fine line between discouraging employees' unprofessional or unfavorable posts online and stepping on employees' right to post them.

GM, for instance, laid out some rather stringent rules for Facebook and other sites, urging employees to think twice before linking up with colleagues online or posting about their employer. The policy warned of "disciplinary action" if workers disobeyed the guidelines.

"If you engage in discussion related to [GM], in addition to disclosing that you work for [GM] and that your views are personal, you must also be sure that your posts are completely accurate and not misleading and that they do not reveal non-public company information on any public site," the memo quotes from the automaker's policy.

If a worker was ever "in doubt" about whether saying something online would violate the guidelines, the policy warned, "DO NOT POST."

It also stated, "Think carefully about 'friending' co-workers ... on external social media sites. Communications with co-workers on such sites that would be inappropriate in the workplace are also inappropriate online, and what you say in your personal social media channels could become a concern in the workplace."

In the memo, Solomon argued that GM's policy is unlawful in several ways. Prohibiting workers from posting anything "misleading" is overly broad, he wrote, since such a restriction could be stretched to cover lawful criticism of working conditions. Urging employees to think twice before friending colleagues is also overly broad, he said, since it could "discourage communications among co-workers." Requiring workers to check with GM managers before posting something would violate labor protections as well, he contended.

Mary Henige, a GM spokeswoman, said via email that the company hasn't changed its social media policy since it was instituted in 2007. "We feel we have a comprehensive policy that informs and guides employees as they navigate the social web, and complies with applicable laws," she wrote.

In the case of DISH Network, Solomon argued that the satellite broadcaster's social media policy infringed on employees' rights when it prohibited "disparaging or defamatory comments about [DISH], its employees, officers, directors, vendors, customers, partners, affiliates, or our, or their, products/services." Solomon said that workers could "reasonably construe" this rule to mean they can't criticize their employer's labor practices.

He also said DISH's prohibition on talking with reporters or bloggers without company authorization infringed on workers' rights under labor law. A DISH spokesperson did not return a call seeking comment.

Target's policy, the memo alleges, overstepped labor law by telling employees they cannot "release confidential guest, team member or company information" via social networks. Solomon argued that such a rule could be seen as forbidding workers from sharing "their own conditions of employment," including wages and working conditions. Molly Snyder, a Target spokeswoman, declined to comment on the memo.

The memo doesn’t chastise every company and its social media policy. In fact, it holds up one rather well-known retailer as a model for all others: Walmart. Every section of the company's recently revised policy is "lawful," according to Solomon. The company's guidelines merely prohibit "inappropriate postings that may include discriminatory remarks, harassment, and threats of violence or similar inappropriate or unlawful conduct."

Walmart's broadest social media rule doesn't violate the law either, according to the memo, even though it's a rule that Internet users love to break: "Be Respectful."

Read the memo:

NLRB on social media