The visualizer accepts a Twitter username and grabs that user's current follower count and avatar from the Twitter API. Next, it finds the nearest square root of the follower count, and resizes and processes the avatar such that one dot = one twitter follower. Dots are always the same size, which means you can put two images next to each other and get a visceral sense exactly how 2,000,000 followers compares with 20,000. The area of a given user's visualization is directly related to how popular they are, which is particularly entertaining where celebrities and atheletes are concerned.
You can try the visualizer and browse other visualizations created with it.