Only 27% of US visitors to twitter are regular users

"Twitter reaches 27 million people per month in the U.S.
55% are female
43% are between 18 and 34
78% Caucasian, but African American users are 35% above Internet average
Average household income is between $30 and $60k
1% of the addicts contribute 35% of the visits
72% are passers by, while only 27% are regular users"
Source: Page 3 of Pear Analytics twitter Study, August 2009