You already asked this once.
You can sue him for anything, but you have to prove your case to win and it's not worth it. Defamation is a very expensive area of the law, and without real damages (I doubt you have them), this isn't much of a case. I sincerely doubt your former employee is telling "everyone" these things, for instance.
Libel is written defamation, and Slander is spoken defamation. Opinions are protected by the first amendment. If this person is couching these statements as opinions, then it is not actionable. If they are being posted as statements of fact and you can prove to the court that you have suffered economic damages as a result (you need proof, the court will not entertain hearsay, speculation or conjecture) then you can sue him for defamation. As my colleagues have pointed out, these actions are expensive to litigate, and due to first amendment protections, very hard to win.
Lying is not against the law. It is protected by the first amendment. So as I stated above, you have to show the court with evidence that you have suffered economic damages and it is a direct result of these posts. You can always have a local lawyer send him a cease and desist letter--that may get his attention. As for your concern about "proving"--that's the whole problem. Truth is a defense to defamation, so if you sue him, he will have the right to conduct discovery into your lives, depose people you know, subpoena records, etc. to show that you in fact have engaged in this behavior. It will probably not turn up anything since you say this is all false, but he has the right to conduct the discovery anyway.
If you've reported it to facebook and they are not helping, and the cease and desist letter does not get his attention, then have the attorney that did the letter advise you on how best to proceed.
We do not have an attorney-client relationship. I am not your lawyer. The statements I have made do not constitute legal advice. Any statements I have made are based upon the very limited facts you have presented, and under the premise that you will consult with a local attorney. This is not an attempt to solicit business. This disclaimer is in addition to any disclaimers that this website has made. I am only licensed in California.
Sign up to receive a 3-part series of useful information and advice about personal injury law.