Maybe you're misunderstanding these policies. Not only do these companies often discuss the pipeline problem publicly (and sometimes offer corporate support to address it), but as rhcom2 pointed out, it's not the only factor.
If it's not the only factor, then there's a chance diverse candidates simply aren't being reached by the typical recruiter strategy. So companies take that hypothesis and incentivize recruiters to diversify their search. Only time will tell if that hypothesis is correct, and this is the only point in the system where race/gender are under consideration. It has no impact on those that apply to FB through a college career fair, or through facebook.com/careers.