According to a NYT article, Twitter's explosive growth did not occur in the expected demographic. Twitter offers a communication medium somewhat like text messaging. Teens' short-message communications revolve around their friends, which seems to be a home run for Twitter. But teens seem to prefer their text messaging over Twitter's broadcasting (multicasting, really) service. Twitter has found more traction, after the initial swarm of early adopters, for promotional reasons, including personal and business promotion. Similarly, adults were late adopters of Facebook-like social networks but now account for much of its recent growth. For a more complete picture of this, here is NYT's article on growth of social networks.