Can I be fired because of a friend's post on my Facebook page?

It depends on what your friend posted and whether your friend is also a coworker. For example, you might be protected if:

• You were fired for discriminatory reasons. For example, if your friend's post invited you to a Seder, and your employer fired you because you are Jewish, that would be illegal. Similarly, if your friend's post reminded you of a meeting of an African American alumni association or a support group for expecting mothers, and you were fired because of your race or pregnancy, that would violate the law. 
• You were fired for discussing working conditions. If your friend is a coworker, and the post involved your jobs, you might also be protected from termination. Employees may not be fired for taking concerted action to discuss or try to change the terms and conditions of their jobs. Employers have faced legal action for firing employees who exchange posts criticizing supervisors or complaining about heavy workloads, for example. 
• Your state protects employees from discrimination based on their off-duty activities. If a state law prohibits employers from taking action against employees because of what they do on their own time, you may have a legal claim against your employer. 
• Your state protects political activity. If your friend's post was part of a political discussion, and your state prohibits employers from discriminating against employees based on their political views or activities, you may be protected from termination. 

These are only some of the legal claims that might be available to you, but your rights will depend on the facts of your case and the laws of your state. The best way to find out whether you might have a wrongful termination claim is to speak to an experienced employment lawyer.