College graduates, young adults, and blacks are the biggest users of Twitter, according to a Pew Survey. Overall, 13 percent of online adults use Twitter. The figure is a larger 16 percent among college graduates, 18 percent among 18-to-29-year-olds, and peaks at 25 percent among blacks. Those least likely to use Twitter are people aged 65 or older (6 percent), whites (9 percent), and rural residents (7 percent).
Source: Pew Internet & American Life Project, Twitter Update 2011