The more Facebook has risen in popularity — culminating in it reaching 500 million followers last June and its Hollywood creation tale The Social Network becoming the number-one movie in America in October — the more legal issues have surfaced. The most high-profile have been the near-constant privacy complaints against the company, which has a history of introducing new, unpopular features that people must opt-out of if they don’t want to submit to rather than opt-in to access.

For outside companies, however, perhaps no legal issue has been more contentious than the issue of firing an employee for something they posted on Facebook. It might be due to a salacious photo or an off-color remark about the business — and it might even have been done on the employee’s personal time — but time and time again, companies have fired workers for “inappropriate” behavior online.

Well, based on a National Labor Relations Board (NLRB) ruling last week, we might be seeing a lot fewer of these incidents in the future.

In what labor officials and lawyers view as a ground-breaking case involving workers and social media, the National Labor Relations Board has accused a company of illegally firing an employee after she criticized her supervisor on her Facebook page.

This is the first case in which the labor board has stepped in to argue that workers’ criticisms of their bosses or companies on a social networking site are generally a protected activity and that employers would be violating the law by punishing workers for such statements.

This specific case deals with a Connecticut ambulance company that fired an EMT, who belonged to a union, for criticizing a supervisor. The company termed the violation as “negative personal attacks against a co-worker posted publicly on Facebook” — something that went well beyond the company’s ban on depicting the company “in any way” on social networks.

Ultimately, however, the legal issue comes down to whether or not what the EMT posted was a protected worker right. Given the fact that a union is involved, those rights are already more clearly defined than they might be otherwise.

And in the opinion of the NLRB, this was a clearly protected right.

Lafe Solomon, the board’s acting general counsel, said, “This is a fairly straightforward case under the National Labor Relations Act — whether it takes place on Facebook or at the water cooler, it was employees talking jointly about working conditions, in this case about their supervisor, and they have a right to do that.”

That act gives workers a federally protected right to form unions, and it prohibits employers from punishing workers — whether union or nonunion — for discussing working conditions or unionization

The ambulance company, naturally, disagrees with this assessment.

Regardless of the details of this case, however, there are obviously still many things that an employee could do on Facebook that would merit termination.

(Off the top of my head, some behaviors would likely include libel, harassment and violent threats, among others). And those without union protection are likely going to have a harder time at receiving re-instatement or fair compensation for unwarranted termination.

But this case, which will go in front of a judge on January 25, will help demarcate the standards that companies must adhere to in cases where the egregiousness of the comments is more murky than blatant libel or harassment. And the likely outcome is that many companies will have to re-evaluate their social media policies to determine if their bans on discussing the company in any way on social networks are overly broad.

For a look back at some of the most high-profile firings over Facebook, check out The Huffington Post’s “Fired Over Facebook: 13 Posts That Got People CANNED.”