A Winter and Spring telephone survey conducted by the Pew Research Center in the first quarter of 2012 found that the proportion of online American adults who use Twitter on a typical day has doubled since May 2011 and has quadrupled since late 2010 and that young adults between the ages of 18 and 24 are the leading the adoption trend.
More details available in the report here which was released on 31 May 2012.