Ars covers a Pew report that says 11% of American’s have used Twitter or a similar service. I find that hard to believe.
What I don’t find difficult to believe is how mobile Twitter users are:
Overall, Pew observes that Twitter users engage news and technology at roughly the same rates as everyone else, “but the ways in which they use the technology—to communicate, gather and share information—reveals their affinity for mobile, untethered and social opportunities for interaction.”
More on the study from Pew:
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.
This might mean Facebook status as well, which makes more sense (and then seems a bit low!).