If you add up the populations of China, the US, Mexico, and Japan you get roughly 1.9 billion people (source: CIA).
Each day, 1.9 billion Coke drinks are served (source: Coca-Cola).
A hundred years ago, the world's population was 1.9 billion people (source: Earth Policy Institute).
Add up the total audiences of the BBC, the Washington Post, the New York Times and The Guardian and you don't even hit one billion people.
The world's population is currently 7.5 billion people, so there's plenty of room for Facebook to grow.