can you be terminated from a job over a Facebook post?

The answer is "it depends" -- it depends on the content of the post. The National Labor Relations Board takes the position that employees have a protected right to discuss the conditions of their employment with other employees of the same employer and potential union representatives. Under its interpretations, you cannot be terminated for such discussions, regardless of the medium in which they occur. However, everything else is pretty much fair game, since an employer does not need a "good" reason to terminate an employee's employment.