Is it legal for an employer to fire an employee because of a Facebook post?

As a general rule, an employee can be terminated for this. In an "at will" employment relationship, an employer can set the terms and conditions of your employment as it deems fit. This includes comments made on social networking sites. Therefore typically, unless you have an employment contract/union agreement that prohibits your discharge for this, your dismissal violated existing company policy or your treatment constituted some form of actionable discrimination, you have no legal recourse.

That having been said, there is an exception regarding the firing of an employee for making negative on-line comments if those comments are about their employer. If the complaining amounted to what the lw terms "protected concerted activity". In such a situation, an employer cannot fire a worker under federal labor law. In some cases, then-line post may be no different from workers gathering around the water cooler to discuss working conditions. The NLRA (National Labor Relations Act) prohibits employers from punishing employees (whether or not they are members of a union0 for talking about wages and/or workplace conditions regading forming a union. The idea is to more easily enable communication between workers so that they can decide if a union is necessary.

Therefore, whether or not you can be fired depends on the circumstances surrounding your comments. If it was simply for negative remarks made to your family and/or friends for no other purpose, then yes you may lose your job over this; if it was to made to other employees for the purpose of improving working conditions, then you are probably protected.

At this point, you can consult with an employment law attorney to find out your rights/responsibilities regarding your particular case.