It is important to remember that Facebook and many other social networking sites are in the business of sharing and monetizing your personal information. Whenever you are provided a service free of charge (such as social networking) it is safe to assume that you, yourself are the product!
Under current circumstances and under current case law it appears that in many jurisdictions employers may:
It is possible after recent campaigns by facebook and one particular case that employers may not:
But remember, if you access your facebook account from your place of employment it is likely that they already have traffic and usage records of everything you have posted.
Be sure to check what your specific rights are in your State or Jurisdiction as laws and expectations of privacy may vary widely.