Source: http://www.emarketer.com/(X(1)S(1gotvubn20o2jfmkm4fab3v1))/Report.aspx?code=emarketer_2000780&AspxAutoDetectCookieSupport=1

US Twitter use will continue to grow through 2013, although usage levels and growth rates will be modest compared with earlier forecasts. Still, Twitter will remain a strong force within the social media ecosystem and a viable venue for marketers targeting youthful, engaged audiences.