You need to sit down with an experienced attorney to discuss the situation.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
The essence of action that constitutes defamation (slander and libel) is untruthfulness. Based just on what you have reported here, it seems as if your co-worker maliciously reported to your employer the truthful fact of a post by you on FB. If the content of the post had been misrepresented by your co-worker in the report to the employer, then you would have promptly corrected that misinformation with a copy of your post to your employer. So, either your employer terminated you for posting, or terminated you for the content of your posting. But the facts you report do not suggest that your employer terminated you based on false information from your co-worker. Based on that alone, there is not a basis in law for any claim or action of defamation. There are other reasons, so, why you cannot make a successful defamation action on these facts, but the lack of untruthfulness is wholly dispositive.
You also cannot likely successfully sue your co-worker for emotional distress or another civil wrong. All civil lawsuits are based on an allegation that the defendant has breached a legal obligation to the suing party. Your co-worker is under no obligation to you to keep confidential or quiet about posts you put up on a public web-site. In all events, the co-worker's action would not come anywhere close to constituting the kind of conduct that must be shown to establish an action for emotional distress. Fundamentally, your co-worker is not legally on the hook for revealing your actions.
It is almost always a terrible idea to post on FB or other social media sites the activities and business of one's employment. Almost all U.S. employees are at will and subject to termination for any reason or even no reason, save for protections against unlawful bias. Employers can be predicted not to appreciate having their business chronicled on the internet by unauthorized persons. There is simply no upside to the practice and plenty of risk, as you have found out the hard way. Even where the employer has not specifically prohibited the practice (and there are reasons for not making that a formal employment policy), in almost all circumstances under the present state of the law, the employer can react to postings by terminating the poster. In the limited circumstances where an employer may be constrained from terminating an employee for internet posting, the employer can usually easily find an alternative lawful basis for accomplishing termination.
Employment is not fair and not not equal. The employer has almost all of the power. The prudent course is to post about your own life, not about your employer's business.
My responses to questions on Avvo are never intended as legal advice and must not be relied upon as legal advice. I give legal advice only in the course of an attorney-client relationship. Exchange of information through Avvo's Questions forum does not establish an attorney-client relationship with me. That relationship is established only by individual consultation and execution of a written agreement for legal services.