Just imagine a 20 person company with 5 women who are being fairly treated: how do they get better gender balance? Even if these five persons were outspoken, how many people could they reach? Not to mention that a true character of a company is only revealed in a crisis (harrasment, known unequal pay, role progression...). This company will never get the breadth of candidates Facebook might to be able to rely on statistics. Without actively pursuing that as a goal, balance never happens.
Positive gender discrimination is a societal tool, a tool to fix unfair social treatment of genders at an early age ("here are pink dolls for you dear, your brother can go play games on the computer" just as a blunt example, but it's a lot more subtle in general).
Run a company that women see as a good place to work, where they won't be harassed and where they're treated fairly. Once the company gets a reputation for being a good place for women to work you'll get a lot more applications from women, and the problem of balance will resolve itself because you can hire on merit if you have a good selection of candidates.
The hard part will be firing people who do things that give the company a bad reputation even if they're great workers.