Twitter vs. Facebook: A Breakdown of 2010 Social Demographics makes interesting use of the Polar Area Chart to break down the demographic data of U.S. Twitter and Facebook users:

"One has over 500 million users, the other just over 100 million. But who are they and what’s their behavior? What’s their value to a brand? How old are they? What’s their education? How much do they make? Just exactly what does the Facebook vs. Twitter landscape look like? Good questions. Here’s how we see it."