Re: Didn't ElReg just have a story about women-only coding camp?
"You could advertise your IT job in Computer Weekly."
Yes you could, but you couldn't then ask Computer Weekly to snip your ad out of any copies that are bought by women, could you? (and even if you could, why would you?)
Likewise it makes perfect sense to advertise a computer-related job on a computer-related Facebook group, and if membership of that group was 90% male, then that would not be a problem, since any interested women could join the group*. But you shouldn't hide the ad from only art of the group based on gender.
*putting aside the further issue of private closed groups for a moment