Facebook ad campaigns heavily favor advertisers slicing up demographics as much as possible. I am going to make the assumption that at the very beginning of the campaign an advertiser created many segments, split by age and gender. After they do that, certain segments are going to respond better and the distribution for those ads is going to increase while the others fall. The alternative is that the advertiser is going to have to jack up the bidding for the non-responsive demographics.
(And for anyone who doesn't know much about digital advertising, despite all of the crazy complexities, all of these systems are basically trying to maximize displays toward people who are going to respond to the ad and eliminate displays to people who won't. The definition of a response at the simplest is a click, but at the most complex is a lead, app install, purchase over a certain order size, etc. In many cases even the deepest pocketed brand advertisers will not be able to out-bid a highly optimized ad campaign for specific inventory.)
On the other side of the internet advertising pie you have Google (excluding Indeed and LinkedIn because I know only a little about their platforms.) An advertiser is going to bid on "driver jobs" or whatever, and Google is going to do the rest. Way back when, before Google collected user demographics, the ad is going to show just for the people who searched it. I'm going to take a guess that some job keywords split 50/50 while others skew heavily toward certain genders or age groups (e.g. 0 females over 65 are searching for manual labor jobs.) With Google's demographic data and machine learning based targeting, what is really going on today, in 2018, is a mystery to me. I would assume that these algorithms are not demographically neutral. They probably learn that males respond more to driving ads. Almost certainly on the display side of things (which Google's less sophisticated ad buyers might not realize they are purchasing) the ads are going to start skewing toward certain demographics.
Solely based on the way Facebook's platform works, I would question if it is ok for any advertiser to place ads on Facebook's platform as Facebook can place the blame on the advertiser even though it is ultimately a consequence of how their system is designed. On Google's side, the same discrimination is likely happening but the advertiser can probably blame Google.
At the end of the day, larger questions persist. How much money should employers be spending on letting employees know that they have a paying job available to them? And, is it ok that job opportunities, which may be superior, and being buried by those that have deeper pocketed employers?
Because of Google's semi-recent launch and expansion of Google Jobs results, which almost certainly will end up heavily monetized, I think these are very important questions.