Facebook is popular as one can interact with friends on a wide scale. On the other hand, teenagers do not use twitter. Most have signed up to the service, but then just leave it as they realise that they are not going to update it (mostly because texting twitter uses up credit, and they would rather text friends with that credit). In addition, they realise that no one is viewing their profile, so their ‘tweets’ are pointless.
Contrary to hype.