Via a tweet from the World Bank Pubs account, the relevant stat nugget can be found in the foreword (page 9 of 211) to the Africa Development Indicators report, by Makhtar Diop, Vice President of The World Bank Group for the Africa region.
Kenya & Nigeria account for 62% of Internet users in Sub-Sah http://t.co/KzR1zMdFZG #Africa #ICT #ADI
— World Bank Pubs (@WBPubs) August 16, 2013