Pew Internet
That's up from the 8 percent of online adults who identified themselves as Twitter users in November 2010. Pew Internet & American Life Project reports 95 percent of Twitter users own a mobile phone, and half of those users access the service on their handheld device. More from the latest report:

As in our previous research on Twitter use, African Americans and Latinos continue to have high rates of adoption of the service. Fully 25% of online African Americans use Twitter at least occasionally, with 11% doing so on a typical day.

Additionally, Twitter use by internet users ages 25-34 has doubled since late 2010 (from 9% to 19%) and usage by those ages 35-44 has also grown significantly (from 8% to 14%).

Frédéric Filloux writes in his May 29 "Monday Note" that Twitter traffic has been flat the last 18 months.

Time spent is eroding: 14 mn 6 sec per user in March 2010 vs. 12 mn 37 sec in March 2011. Contrast this to more than 6 hours spent on Facebook. Despite occasional news cycle-triggered traffic outbursts (the Spring unrest in Arab countries is a good example), such spikes don’t really translate into audience gains. As for the number of accounts, half are idle. And, as usual on the internet, the usage is extremely concentrated: 10% of all users account for 90% of the twits.