A federal agency recently brought a complaint against a Connecticut medical-services company for allegedly firing an employee over a Facebook post.

But it may not be wise to take that as carte blanche to go online and type in exactly what you think of your boss.

The general rule is that employers can still fire workers for off-color or unsavory things they say when blogging (or facebooking or tweeting) on the job or about their job.

"It would be a mistake for people to say that, 'Just because I'm on Facebook, I can say whatever I want,'" said Charles Cohen, senior counsel at the Morgan Lewis law firm in Washington, D.C. "That's the main point."

There are exceptions to that rule. Union employees with a contract are often protected from being fired without "just cause." Being fired because you disclosed your religion or sexual orientation on a blog is likely illegal.

Finally, organizing a union has long been protected under federal law--which is the right that the National Labor Relations Board is seeking to expand to social-media sites in its new complaint filed against American Medical Response.

The complaint says (PDF) that AMR employee Dawnmarie Souza "engaged in concerted activities with other employees by criticizing respondent's supervisor...on her Facebook page" on November 8, 2009. Souza was fired on December 1, 2009. (An administrative-law judge will hear the NLRB's allegations on January 25, 2011.)

Note the key phrase "concerted activities." That's a reference to a 1935 federal law called the National Labor Relations Act. Section 7 of the law says employees have the right to organize labor unions and "to engage in other concerted activities for the purpose of collective bargaining or other mutual aid or protection."

CNET has obtained copies of Souza's Facebook posts. In one, she called her supervisor a "dick." Another said "he's a scumbag as usual." One reply said: "I am sorry, hon! Chin up!"

Invoking scatological language when describing the relative merits of your job is, of course, a time-honored American tradition. So is firing employees who do it to your face. But is a Facebook post calling a supervisor a "scumbag" necessarily done "for the purpose of collective bargaining or other mutual aid or protection"

Perhaps not. Or, at the very least, it may not be exactly what the drafters of that 1935 federal law--billed as promoting "the flow of commerce by removing certain recognized sources of industrial strife and unrest"--had in mind.

A 1978 Supreme Court case, Eastex v. NLRB (PDF), said employees could distribute a union newsletter (during nonwork hours) that took explicitly political positions. Federal law, a majority of the justices said, encompasses that newspaper distribution as a type of "concerted activity."

But the majority also acknowledge that "some concerted activity bears a less immediate relationship to employees' interests as employees than other such activity." That kind of activity, the justices said, "cannot fairly be deemed to come within the 'mutual aid or protection' clause" and could be legally punished by an employer.

"We're trying to adapt that law to the modern workplace," Tony Wagner, a representative of the NLRB for new media, told CNET. "This is really the first case we're aware of that directly involves a social-media component."

Complicating the complaint that the NLRB filed against the Connecticut company is that it's not just about a few Facebook posts: Souza allegedly requested union representation during an internal AMR disciplinary process and was refused. That dispute apparently is what led her to vent online.

The NLRB's complaint against the company stresses that point, alleging that AMR "has been discriminating in regard to the hire or tenure, or terms and conditions of employment, of its employees, thereby discouraging membership in a labor organization," in violation of the law.

For its part, AMR says the case is really unrelated to Facebook.

"The real issue here doesn't have anything to do with Facebook," said John Barr, a partner at the Jackson Lewis law firm who represents AMR.

Even in the absence of the Facebook posts, "the employee most likely would have been fired," anyway, because of repeated complaints about her "rude and discourteous behavior."

AMR doesn't monitor Facebook posts, Barr said. Instead, "the way AMR finds out about it is that one day, the supervisor has personnel from the fire department call him up and say, 'What's this on Facebook'" Moreover, he added, AMR has not disciplined other employees for their out-of-school Facebook conversations.

"It's the company's position that the Facebook posting was not protected activity" under Section 7, Barr said.

While this appears to be the first time that the NLRB has taken action against an employer based on social media, it has addressed the topic before. In a memorandum sent to Sears in December 2009 (PDF), the agency said the company's Internet policy did not violate the law.

Sears' policy prohibited the "disparagement of company's or competitors' products, services, executive leadership, employees, strategy, and business prospects" through social media.

That's remarkably similar to AMR's Internet policy, which says: "Employees are prohibited from making disparaging, discriminatory, or defamatory comments when discussing the company or the employee's superiors, co-workers, and/or competitors."

This is hardly the first time someone was fired for Facebook comments made at work (or about the employer), and it won't be the last.

Last year, a U.K. employee was fired for saying her job was boring. So was a Swiss insurance worker who was fired after claiming that she was so sick, she couldn't use a computer but was caught logging on to Facebook from home.

"We don't have the contours yet, vis-a-vis, this medium," said Cohen, the employment lawyer in Washington, D.C. "We have a lot of analogies from the past."