From the Pew Internet and American Life Project:

“As of December 2008, 11% of online American adults said they used a service like Twitter or another service that allowed them to share updates about themselves or to see the updates of others. Just a few weeks earlier, in November 2008, 9% of internet users used Twitter or updated their status online and in May of 2008, 6% of internet users responded yes to a slightly different question, where users were asked if they used “Twitter or another ‘microblogging’ service to share updates about themselves or to see updates about others.”

Full report here.