"We can't provide legal advice, but we can show what we have found to be acceptable and unacceptable, and explain why," she added. "We're applying a 77-year-old law to forms of communication that didn't exist when it was written."
In Solomon's two previous memos, he focused on recent cases in which companies fired employees for using social media to discuss working conditions. The agency has attempted to highlight a large number of cases with a "variety of scenarios," Cleeland said.
The NLRB can order companies it has found to be in violation of the labor act to rehire fired employees or pay for lost wages. The board can also order companies to change their social media policies, Cleeland said.
In the May 30 memo, Solomon found that a company's prohibition on employees commenting on legal issues was illegal because it could restrict workers from commenting on employment claims. A policy requiring workers to adopt a "friendly tone" online, to avoid picking fights and avoid inflammatory topics was also a violation of the labor law because discussions about working conditions "have the potential to be just as heated or controversial as discussion about politics or religion," Solomon wrote.
However, Solomon found a Wal-Mart provision focused on obscenity and harassment to be legal. That policy encouraged employees to avoid posting information that "could be viewed as malicious, obscene, threatening or intimidating, that disparage customers, members, associates or suppliers, or that might constitute harassment or bullying."
One of the messages of the latest NLRB memo is to avoid ambiguous language in social media policies and spell out situations to avoid, said Mayer Brown's Goodman. Companies should give "concrete examples" of social media activity that is allowed and prohibited, she said.
The new memo leaves questions about how companies can prohibit employees from releasing confidential information on social-networking sites, she said. The memo acknowledges that companies have the right to protect "certain" confidential information, but doesn't spell out what that is, she added.
The NLRB seems to be on a "journey" to address social media in the workplace, Goodman said. The three memos, which don't lay out specific rules, may be confusing to many companies, she added.
"They could just tell you the rules, but that would be too easy," she said. "They make you figure them out yourself."
Barry Bendes, a partner at the Edwards Wildman law firm, disagreed, saying the newest memo gives good examples of what companies should and should not do.
"What they're doing is giving employers guidance in an area where it was needed," he said. "Without the guidance you had no idea whether some of the provisions in your policy would be found to be unlawful."
Sign up for MIS Asia eNewsletters.