Why is Twitter irrelevant to a 17 year-old girl?

Over Christmas, I attempted to sell Twitter to my 17-year old sister. Three years prior, I successfully encouraged her to sign up for Facebook, arguing that her early adoption will make her the idol of her freinds once the site catches on (and it did, and she was).

She now uses Facebook for all communication. She has no email account–posting on friends’ walls is a more efficient form of communication for high school students. Every status update signals a flurry of comments and discussion.

So when I attempted to explain Twitter–how you can have status updates, but it’s only 140 characters, and how you can follow people…I quickly realized how stupid I sounded. Twitter provides no value to her. The character limit is even more absurd from this perspective. Posting links that require you to use TinyURL–this makes no sense for someone that lives on Facebook where photos, videos, and links can easily be shared with ease.

I then shifted to the argument that you can follow people you do not know (e.g., RealShaq, CNN, Scoble). But you can do this on Facebook via pages and public profiles. In fact, her teacher uses public status updates to communicate with her class.

In short: can someone give me a compelling value proposition for users of Twitter? Specifically, why it seems to be so popular among technophiles/bloggers but no one else?