According to the National Labor Relations Board,
complaining about your boss on Facebook is legally protected. National Labor Relations Act forbids employers from firing or otherwise punishing workers for discussing working conditions, and a new case seeks to extend that protection to Facebook. A woman was fired for posting negative comments about her supervisor, and the Labor Board has filed suit, calling this a violation of the Act. I think the Labor Board is doing the right thing, but I also think it's foolish to complain about your boss on Facebook. Even if they can't legally punish you, it is not a wise move.
But here's where things get really interesting, from the New York Times article I linked to above:
The labor board said that her comments “drew supportive responses from her co-workers” and led to further negative comments about the supervisor…
Marshall B. Babson, a member of the National Labor Relations Board in the 1980s, said a broad company rule that says one cannot make disparaging comments about supervisors is clearly illegal under labor law. But he said an employee’s criticizing a company or supervisor on Facebook was not necessarily protected activity.
“There will arguably be cases where it is not concerted activity,” Mr. Babson said, suggesting that if a worker lashed out in a post against a supervisor but was not communicating with co-workers, that type of comment might not be protected.
If the Facebook conversation involves several co-workers, however, it is far more likely to be viewed as “concerted protected activity,” he said.
If Babson's interpretation is correct, this would seem to mean that whether or not my online activity is legally protected depends partially on the response it gets from others. If I get fired for calling my boss a jerk on Facebook, and I decide to sue, my case is stronger if lots of people I work with respond in agreement. That seems a little backwards to me.