Re: Is it discrimination
"If one in 100 guys is a truck driver and just one in 1600 women is (numbers roughly correct for the US), then, since you're paying to have your job ad shown, you only pay to show it to guys, because otherwise you're wasting half of your money by paying to show the ad to people who are 16 times less likely to go for it."
That's awful reasoning. You're assuming that the choices are:
a- target women and get one in 1600 people interested
b- target the general population and have one in 850 people interested
c- target men and get one in 100 people interested
So choose option (c)
But actually if I have a powerful ad platform like Facebook or Google that can finely target a given demographic, I would choose option (d) - target groups of people who are probably truck drivers. Based on how good the FB / Google / other ad networks' algorithms are, it's not going to be a 100% match, but it will certainly be better than 1/100.