Here’s a very good PowerPoint Presentation from The Pew Research Center that will tell you everything you need to know about Twitter users at this point in time.
One point that Pew references in their research that isn’t in this presentation comes from a Harvard study. Harvard took a random sample of 300,000 Twitter accounts and found that 10% of the people who use Twitter, account for 90% of the Tweets.
So, while Twitter is growing, especially among young people it’s still a long way away from being a good place to sell products or services.