Targeting peopel who are already truck drivers
Well that's fine if you are looking to fill a position that requires experience and you don't want to train them. What about entry level jobs. Let's say cooks and servers in a diner. If the diner has been around a decade and only had one female cook and one male server and dozens of the other sex, an owner on a tight budget probably considers it a waste of money to advertise both positions to both sexes.
If there was a way to target "people who would consider working as a cook" and "people who would consider working as a server" obviously you'd MUCH rather target that way than by male or female. But that's not the reality, you can only target those who already have those jobs, which greatly limits your reach (and is a zero sum game if everyone does it)
Plus, it isn't like Facebook would be your ONLY job posting. Maybe you advertise on a community website that offers flat rate help wanted ads, which are open to anyone to see. Maybe you post a sign on your door that you need a cook or a server. Maybe you take out a classified ad in the local paper or on Craigslist.
Is it so terrible to discriminate (without any intention to restrict the positions by sex) in ONE avenue of advertising, when others are equal?