It's libel when it's in writing. But to have any claim, you have to have damages; i.e., someone believes this and your suffer financial loss. Report her to Facebook and block her.
To questioners from West Virginia & New York: Although I am licensed to practice in your state, I practice on a day-to-day basis in Massachusetts. I answer questions in your state in areas of the law in which I practice, and in which I feel comfortable trying to offer you assistance based on my knowledge of specific statutes in your state and/or general principles applicable in all states. It is always best, however, to work with attorneys and court personnel in your own area to deal with specific problems and factual situations.
I practice law in NY so I can only speak generally. As the defamation is in writing, it is a libel. You need to speak to a lawyer in WV who handles defamation suits. Sounds like it would be libel against your company if people can easily identify it through you. As to you, it sounds more likely non-actonable opinion. But you certainly might want a lawyer to write a letter for you telling this person to stop the postings and any other communications.
Report this to Facebook. A law suit is likely not appropriate in this situation because it will be difficult to prove damages. I would speak with an attorney and send her a cease and desist letter if you are concerned that she will continue to libel or slander you or your new company.