In 2009, there will be 18 million US adults who access Twitter on any platform at least monthly. That represents a 200% increase over 2008 levels. Usage will reach 26 million US adults in 2010, a further 44.4% climb.  This forecast counts 11.1% of Internet users as Twitter users this year, a figure close in line with Harris Interactive and Pew Internet & American Life Project estimates of 13% and 11%, respectively, in spring 2009. In 2010, eMarketer expects 15.5% of all US adult Web users to use the microblogging service via any platform.  More stats and charts at eMarketer.