This is an area of the law that is, unfortunately, unsettled due to the method of speech. With the increased reliance on social media, employers have become very sensitive to thier reputations in the community. Large corporations monitor facebook, myspace, and twitter to address issues that appear in those social network sites.
Although an employer may not "technically" be able to restrict your speech, the law and the real world are often at odds. In Pennsylvania, you are an at-will employee, meaning the employer can generally terminate your employment for any reason or without giving a reason. So, the question that I would pose back to you is this: Is what is to be said so important that you may lose your job?