In a way, this is similar to the argument the FBI made to assert that GPS trackers didn't constitute a search because they could just follow someone around and get the same data. The argument against that, and this, was that the technology is sufficiently more powerful and sufficiently out of step with people's expectations that it's a difference in category, not degree. (IANAL, if that's not obvious)

The difference with the new systems is that, while I might happen to come across an ad in traditional media because I woke up early or I might happen to be reading a paper I normally don't, there's practically no chance whatsoever for me to accidentally see an ad in Facebook's systems that's not targeted at me. That's a categorically different situation because I'm being explicitly excluded from that content based on factors specified by the employer, not just by luck or personal preference.

Also, the "implicit filtering" of traditional media only passes the bar because it can't actually be proven that the employer is filtering based on age, gender, etc., which isn't the case with an explicit filtering system. Yes, employers can still skirt the system with the traditional media targeting, but they're still illegally discriminating based on age or other factors.