Earlier quoted context omitted.
If you were to only recruit from single gender schools then I personally would have an issue with it, same as this issue. However, if you are recruiting from many universities and some happen to be single gender, that is not discriminating.
What if the companies that were placing job ads on Facebook were also placing job ads on other platforms that weren't targeted to men only? Playing devils advocate here...
But in general, there are classes of ads that you just can't target based on certain classes, period. Even if you hypothetically might have the ability to target a counterbalancing ad somewhere else. Facebook repeatedly has been in hot water over this, and both they and the people using their ad platform to do this stuff need to get more than a slap on the wrist, in order to deter others from trying the same in the future.