More to explore:
15% of online adults in the U.S. now say that they have used Twitter in the past and 8% told Pew that they do so on a typical day. Overall Twitter usage has actually remained pretty stable since early 2011 (the slight changes over the last year are safely within the margin of error). What has changed, though, is the number of daily active users, which increased from 4% in May 2011 to 8% today.
8% Of U.S. Online Adults Now Use #Twitter Daily
[Study] Twitter User Insights for Marketers
8% of Online Adults Use Twitter Every Day [Social Media study]
Pew: Twitter demographics & usage over time (2012)