Sorry to say but Chris Cox is not quite correct. If Facebook wanted to staff an Engineering Office in Nigeria it would find a lot of talent. There are, after all, a lot of Nigerian engineers in the Silicon Valley area, in New York, Boston, London. If they wanted to widen the pool to staff the Nigerian office, there are lots of Ghanaians, Cameroonians, Liberians, Sierra Leonian talent.
Just look at Telecoms which is now a $5–10 billion industry in Nigeria. MTN, Glo, Airtel, Etisalat all have loads of Nigerian telecom engineers. Service providers like Ericsson, Huawei and Alcatel have loads of Nigerians and West African engineers.
Truth is that for tech the Nigerian market for facebook is tiny. Not much ad revenue compared to other places. So they operate accordingly.