This would be another top-down approach (like the rice question you've asked).
I would recommend the following approach:
Understand the approximate population of Sierra Leone
Split the population by age (The logic here is that social media usage changes most by age) Note: Most countries in africa have a narrow population pryamid...meaning there are a lot more young people than old people.
Estimate the % of each age band with eletronic devices and internet (You should note income splits by age here: i.e. the youngest and the oldest might not be able to afford these)
Estimate the % using social media by age band (i.e. teenagers, young adults, adults, and elderly) (you should make note of 1) Where these groups are (i.e. city vs rural)...i.e. if they're rural (higher % elderly), they're less interested in social media
For the same age bands: estimate the daily frequency of social media use, for those using it
Multiply out and viola :)