Randall Munroe writes the legendary XKCD comic strip online. Besides being hilariously funny he also produces a few fascinating infographics. His latest is below, a to-scale map representation of the relative sizes of various online communities.
A scale map of the size of online communities.
Go here for a large version.
Best trivia I've learned while working on this: "Man, Farmville is so huge! Do you realize it's the second-biggest browser-based social-networking-centered farming game in the world?" Then you wait for the listener to do a double take.
This is the second of these maps he's done. His first was in 2007. Empires rise and empires fall.