Facebook can't gain market traction in Africa if there's no decent internet, so regardless of whether FB will have a direct financial benefit in the connection, they will have an indirect one.

Assuming FB wins over either Chinese social networks or African-developed and operated ones of course.

It is just like overland routes. Imagine you have a bunch of delivery trucks on a highway and you are paying tolls along the way. At some number of trucks it makes sense to just pave your own road next to the highway.

FB is planning 10-15 years ahead because Africa is the next part of the world that will develop a middle class.

Presumably they own some of the bandwidth.

You don’t really prioritize traffic on a wave. Either you own the wave or nah.