The study analysed more than 11.5 million tweets from across the continent during the final three months of 2011.
Kenya came in second place, generating 2.48 million tweets – followed by Nigeria (1.67 million), Egypt (1.21 million) and Morocco (0.75 million).
The average age of an African tweeter ranges from 20 to 29 years old, which is significantly younger than the global average of 39.
The majority (57 per cent) of tweets across the continent are posted from mobile phones, rather than computers.
Twitter originated as an SMS tool and the format of text messaging inspired the founders to set the 140-character limit.
Africa is a big target for the American microblogging company, which is focused on making the site as easy to use as possible on feature phones.
Thousands of people in less digitally advanced countries, such as Cameroon and Sudan, are also now using the service to disseminate information.
Mark Flanagan, Portland's partner for digital communications, said: "One of the more surprising findings of this research is that more public figures have not joined Africa's burgeoning Twittersphere.
"With some notable exceptions, we found that business and political leaders were largely absent from the debates playing out on Twitter across the continent. As Twitter lifts off in Africa, governments, businesses and development agencies can really no longer afford to stay out of a new space where dialogue will increasingly be taking place."
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