If Facebook is attempting to meet diversity quotas, which certainly exist, by pulling from a limited pool of qualifying candidates, in some scenarios, they will necessarily need to employ lower qualified candidates.
I'm NOT NOT NOT saying that women in general cannot code, or produce lower quality code, or anything of the like.
I AM saying that there are fewer female developers than male developers and, assuming an equal distribution of skill level between the two groups, there will necessarily be fewer highly qualified female coders than highly qualified male coders.
Thus, if Facebook wants a 50:50 male-to-female ratio, they will necessarily have fewer highly qualified female developers IF the number of positions to be filled by females exceeds the number of highly qualified female applicants, but the number of positions to be filled by males does not exceed the number of highly qualified male candidates.