Richard Florida did some analysis of the 500 most followed on Twitter, and discovered that most lived in the US, in Los Angeles and New York:
Richard Florida, Where the Twitterati Live
One explanation for America’s dominance is that it had a head start: Twitter was created and launched in the U.S. Most top U.S. accounts were registered earlier than those in other countries, and there is a clear relationship between how long an account has been active and how many followers it has accrued. It’s likely that American dominance will weaken somewhat as tweeters in other countries catch up. But that is clearly not the whole story.
And the use in Brazil is very interesting, and unexplained, here.
I blame Hollywood. And the US’s general emphasis on social networking to find a job nowadays.