[...] Aaron Smith, a senior research specialist with the Pew Research Center, said Pew Internet studies show Twitter's demographics are also a key.

"When you look at Twitter in comparison with some of the other online tools, it skews a bit young; it's fairly racially and ethnically diverse" and generally upscale, Smith said.

"It offers the ability to tap into politically engaged individuals," he added, "and give them a sense that they have a personal connection with a candidate or a campaign they feel very strongly about."

Twitter demographics

A Pew study this year showed that African Americans and Latinos use Twitter at more than twice the rate of white Americans. Twitter users are also more likely to be men, under 50 and with a college education, the study showed.

In addition, about 13 percent of online adults use Twitter - up from 8 percent last year - and fully half of them access the site through their mobile phones, the study showed.

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