Can a defamatory comment posted by an employee on his or her Facebook page be used as evidence in the employee's disciplinary proceedings?

Yes, provided that the evidence is obtained by lawful means. For example, hacking into an employee's Facebook account to obtain evidence of defamatory content would be unlawful, whereas if the employer accessed the content legitimately using its own Facebook account (or, more likely, the account of one of its employees with his or her consent), this would be lawful. In the absence of such evidence, the employer is likely to encounter difficulties in bringing disciplinary proceedings and imposing any sanction.