Of those 100 million people, 80 per cent are accessing the platform via mobiles on a continent with very low internet penetration and smartphone ownership, it said.
Facebook's global mobile advertising revenue jumped more than 150 per cent in the second quarter, accounting for about 62 per cent of overall ad revenue.
"There is a fantastic opportunity for business as well if you are local or a global business. Knowing that there is all these people now in Africa that you can connect with, that is exciting as well," said Nicola Mendelsohn, Facebook's vice president for Europe, Middle East and Africa.
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Seven in 10 African consumers access the internet through their mobile phones, compared with 6 per cent who use desktop computers, according to an Ericsson study earlier this year.
Facebook would be pouring more resources into Africa to understand how consumers use its product and was considering opening an office on the continent in 2015, Mendelsohn said, without saying where the office might be.
The company is customising products to fit the needs of individual countries, she added. For example, it can now target users based on their internet speeds.
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Facebook has more than 1.5 million active advertisers and more than 1.3 billion active users globally.
Mendelsohn could not say how much Africa contributes to advertising revenues but said the Europe, Middle East and Africa region makes up 30 per cent of global revenue.