Paul Butler, an intern at facebook, decided to map the connections between ten million or so facebookers. I won't go into the details of the how he did it but the map of the world you're seeing above is made up of the densities of connections between friends in different cities and countries. In other words, you'd expect strong facebook friend connections within a city and between capitals so that is reflected in the map with bright spots and brighter connecting lines.
Relationships between countries are done with curved lines. It's interesting for me to see Australia's strong connection to Indonesia - especially coming out of Perth.
You can read more about this fascinating project and view a high res version of the map from facebook notes page.