Whenever I talk to teenagers, they laugh uproariously at the all-too-popular media fancy that they’re tweeting away, voicing their firmly held likes and dislikes in 140 characters. Not so, they always say. Why use Twitter when you can simply text your friends, as kids do nearly 24 hours a day? In fact, when I spent a day with a bunch of high-schoolers who made up this year’s Summer Movie Posse, they said they didn’t know anyone their age who used Twitter…