Go ahead, bash your boss on Facebook

January 30, 2013, 5:00 PM UTC

FORTUNE — Old laws and new technologies make strange bedfellows. The latest proof: In an effort to catch up with the times, the National Labor Relations Board has turned its attention to employees’ use of social media, with surprising results. It turns out that the National Labor Relations Act, passed in 1935, which protects workers from suffering reprisals for trying to organize a union, can also apply to people who are just venting on Facebook.

So says the NLRB’s decision in a recent case, Hispanics United of Buffalo, Inc. Closely watched by companies’ legal departments because it was “one of the first cases to address a discharge from employment on the basis of social media activity,” the ruling sets a precedent, notes Jesse Dill, a labor attorney at Arnstein & Lehr in Milwaukee. “Unfortunately, things did not turn out well for the employer.”

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Here’s what happened. At Hispanics United of Buffalo, a nonprofit set up to help victims of domestic violence, an employee named Lydia Cruz-Moore persistently criticized the work of colleagues, including one named Marianna Cole-Rivera. In October 2010, Cruz-Moore texted Cole-Rivera, saying she was going to take her complaints to the organization’s executive director. Cole-Rivera reacted with a Facebook post that read: “Lydia Cruz, a coworker, feels that we don’t help our clients enough at [Hispanics United]. I about had it! My fellow coworkers, how do u feel?”

Four colleagues responded with their own posts, taking Cole-Rivera’s side. So Cruz-Moore griped to their boss who, a couple of days later, fired Cole-Rivera and all four of her allies.

The ruling against Hispanics United seems odd for a couple of reasons. First, while the NLRA protects employees’ activities that are for their “mutual aid and protection,” there was no suggestion that the people who were fired in this instance planned any activities at all — or, as the dissenting opinion in the ruling put it, “[T]he mere fact that the subject of discussion involved an aspect of employment … is not enough to find concerted activity for mutual aid and protection.” Says Dill, “Many people take to social media to vent their employment frustrations, but most have no intention of acting any further.”

And second, many companies — especially those with few or no unionized employees — never paid much attention to the NLRA, since most of the law’s provisions relate specifically to unions. “The NLRB is really coming out of left field and surprising employers with this,” Dill observes. “Companies now have to re-examine their social media policies in this new light of whether Facebook posts, tweets, and other online activities might be protected.”

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At least for now. Hispanics United has announced plans to challenge the decision, which could be overturned by a court of appeals. In the meantime, Dill notes, it’s still not smart to badmouth a boss or a colleague via social media, if for no other reason than that it’s a red flag to prospective employers. “It’s so easy to vent on Facebook,” he says. “But once you put something online, it’s hard to take it back. It’s out there forever.” Too true.