Facebook has opened its first headquarters in Africa as it looks to add to its existing 120 million users on the continent.

The new office in Johannesburg will focus on growing markets in Kenya, Nigeria and South Africa, it says.

About one in five people in Africa have internet access, but almost double that figure are expected to have mobile internet connections by 2020.

About 80% of those who use Facebook in Africa access the site by mobile phone.

"This is one of the places where our next billion users are coming from," Facebook's vice-president for Europe, the Middle East and Africa Nicola Mendelsohn told Bloomberg news.

Facebook says it will also be looking to boost growth in Senegal, Ivory Coast, Ghana, Tanzania, Rwanda, Uganda, Zambia, Mozambique and Ethiopia as part of its increased presence in Africa.