Most teens don’t use Twitter because it doesn’t enable them to do anything they can’t already do elsewhere, which is the same reason most adults don’t use Twitter. It has nothing to do with any teen-specific concerns like texting plans or safety. It comes down to something more simple: delivering value beyond Facebook and MySpace — a competitive moat that Facebook is bridging one move at a time, from the Everyone button to the acquisition of FriendFeed to the centrality of the stream itself.
The question of “Why Don’t Teens Use Twitter?” is the question of “Why Doesn’t Everyone Use Twitter?” The answer, it would seem, is both obvious and heretical … maybe Twitter isn’t for everyone.
Source: here