The simple question is: Does everyone have an equal opportunity to this position or not? If you can't even see the ads then the answer is no, and so it's a violation. If there were different ads for different groups but everyone still had the opportunity to see, apply and acquire the position then it's fair game. Perhaps Facebook could improve job ads to allow for more specific targeting but always have a fallback ad…
I agree. As a female software developer, I would like to see those ads so I can get better jobs. If I don't even see the ads, then I would not even know about opportunity. Without opportunity, I cannot really progress. If only men see the ads, apply jobs and get interviews, then the employers may think that they shouldn't advertise to women. This is just circular thinking.
If the employer targeted the ad just to save money by advertising to a demographic that is more likely to be responsive, that's a different story. Then you might as well argue that you are denied opportunity when, instead of some job ad, you are shown an ad for Coca Cola because you "liked" Coca-Cola at some point.
You're not really entitled to see job ads at all, if you look for a job, you can reasonably be expected to find the ads yourself instead of waiting for them to show up randomly.
Lastly, it's usually garbage jobs that need to be advertised to people who aren't even actively looking. What if that "opportunity" turned out to be exploitation? Would you then fault Facebook for displaying you the ad?