In the beginning, Facebook simply connected college kids on campuses. The "population" was small and consisted of people like Zuckerberg himself, who famously didn't graduate from Harvard. Then Facebook connected everyone in America. And now, as it rolls out globally, Facebook has connected its first 1 billion people and is well on the way to drawing in a second billion.
But on the scale of billions, Zuckerberg has been forced to think about exactly who is inside that second, third or even fourth billion people.
It's obviously not college kids in America and Western Europe, the people who live in the most wealthy nations.
Rather, a huge chunk of new users will come from developing countries where their incomes are much smaller. The world's poor, in other words. This Wikipedia map (not the Zuckerberg map - that's below) shows that three entire continents - South America, Africa and Asia - are dominated by people who make on average less than $15,000 a year.
Wikimedia CC
The problem for Zuckerberg is, how do you connect these people with Facebook when billions of them won't be able to afford a phone, let alone a desktop computer?
His solution is to persuade wireless carriers to provide basic mobile phone access for free, and to give them coverage with an array of drones, satellites and lasers.
That sounds massively ambitious, until you see the map Zuckerberg is working from. As he points out in a white paper for Internet.org, the nonprofit he founded to bring global mobile Web access, most people are already covered by cell phone service:
Our research has shown that approximately 80-90% of the world's population lives today in areas already covered by 2G or 3G networks. These environments are mostly urban or semi-urban, and the basic cell and fiber infrastructure has already been constructed here by mobile operators. For most people, the obstacles to getting online are primarily economic.
Here is what Zuckerberg's map looks like, with basic 2G phone coverage in light blue, and darker/red indicating areas with high-quality 4G coverage and Wi-Fi access:
Right now, in the Zuckerberg world view, billions of people in Africa and Asia basically don't "exist" because they don't have access to the Web - yet.
That's not because they don't have satellites, lasers and drones flying over their villages. As Zuckerberg makes clear, the problem is economic, not technical: "For most people, the obstacles to getting online are primarily economic."
By solving the "economic" problem - and giving the poor free access to the Web - Zuckerberg is basically making a gigantic, global startup seed fund investment. He's hoping that of those billions, maybe millions will use their access to create new businesses, new jobs, and new trade markets.
It's possibly the most ambitious third-world infrastructure project on the planet right now. Let's hope it works.