The National Labor Relations Board alleges that the woman's comments are protected speech under federal law that allows employees to discuss their jobs and working conditions with co-workers.
The case could set a precedent as more workers use social networking sites.
A spokesman for American Medical Response of Connecticut says the employee was fired because of complaints about her work. But the company defends its policy prohibiting workers from depicting the company in any way on the Internet.
An administrative law judge is expected to hear the case in January.