Lets compare face book to other silicon valley tech industries. I know in Santa Clara county blacks make up 2.4 % of the population and make 2.1% of the PhD holders.
I went to prominently black church in San Jose and people there worked at places like Locke heed, BAE(yes there is BAE in Santa Clara) HP, IBM et. So lets see why there is such a low rate of blacks working at face book.