Can an employee legally discuss workplace issues on the Internet?

A disgruntled employee went on Facebook and described a salary disagreement with his boss, Dr. Labor. Dr. Labor found out about this and promptly fired the employee for violating an office policy on not discussing office-related issues on social media. The now-former employee filed a lawsuit against him with the National Labor Relations Board (NLRB) alleging that his former boss violated his First Amendment rights.