The digital media trackers at the Pew Internet & American Life Project have determined that 8% of Americans now use Twitter. Pew did not say how many of the are based in digerati sectors such as New
York, San Francisco, and L.A., but they did say it tends to be more popular with "young adults, minorities, and those who live in cities." Details from the study, which is the Pew's first on Twitter
usage, can be found here: http://pewinternet.org/Reports/2010/Twitter-Update-2010/Findings/Overview.aspx. Okay, so that was considerably more than 140 characters, but you're still reading, aren't
you?