There is no place for discrimination on Facebook
True; it's equally vile wherever you look.
For all Facebook's faults, however, this isn't a new phenomenon. Employers have always targeted advertisements - knowingly or unknowingly - by the choice of publication in which those advertisements were placed.
More interesting is that potential employers continue to believe they have any meaningful influence over the suitability of the candidates they eventually hire. Many selection techniques pick the "right" candidate less than 50% of the time*. The most commonly used (the unstructured interview) delivers a good candidate around 31% of the time. So the arguments about whether any form of discrimination is "justified" are pretty much futile. As long as you can get some evidence that people have approximately the right skill set, you might as well just pick them at random unless it's the kind of job for which it's worth putting people through a rigorous assessment centre process (where you can get the likelihood of a good hire up to 68%).
*Fundamentals of Human Resource Management: Managing People at Work
By Derek Torrington, Stephen Taylor