Having lived and worked in Nigeria, I can say that there is enormous potential but with net access costing $100 per month, it is out of reach for the average person. Electricity supply is sporadic and generators/diesel is expensive. So if you take a population of 160 million and then whittle it down to people who can afford net access and electricity, the actual market becomes much smaller. We have not even touched on the issue of online payment systems and lack of postal service for physical delivery of goods, which would reduce the business model selection.
You might say that there is opportunity to provide cheaper net access, electricity and other infrastructure items but these are heavily licensed sectors that need a huge amount of capital just to be in the game, which means the price to the end user will probably be high as you need to justify a return.
Plus you have to deal with running a business in an environment where you need to pay for basic infrastructure (electricity, security etc etc) which increases costs further.
There are problems to be solved and startups can find a niche in Africa but it means working from a local perspective and not just transplanting a concept from a developed market where the needs and requirements of the market as well as the cost base are radically different.