The Pew Internet for the American Life Project is out with new data about Twitter ...
"Some 19% of internet users now say they use Twitter or another service to share updates about themselves, or to see updates about others. This represents a significant increase over previous surveys in December 2008 and April 2009, when 11% of internet users said they use a status-update service."
However, what's also notable is that Twitter is stronger on average with the younger audience - the same one that TV nets and advertisers covet. Overall it's younger than Facebook, which is graying. Still, pound for pound I am sure Facebook has more consumers in the same demo given its girth ...
"Internet users age 18-44 report rapid uptake of Twitter over the last nine months, whereas internet users ages 45 and older report slower adoption rates. For example, 37% of internet users age 18-24 use Twitter or another service, up from 19% in December 2008.
The median age of a Twitter user is 31, which has remained stable over the past year. The median age for MySpace is now 26, down from 27 in May 2008, and the median age for LinkedIn is now 39, down from 40. Facebook, however, is graying a bit: the median age for this social network site is now 33, up from 26 in May 2008."
This also explains why Twitter users like current events and culture. For more, see the PDF from Pew embedded below.

PIP_Twitter_Fall_2009.pdf (451 KB)
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