Ask HN: how to reach teenagers with a social network?

Hi HN,

As far as I know, the easiest audience to reach by social networks, and subsequently the first audience which is reached, is 20/30-somethings. Facebook would be a bad example as it was first pitched at college students, but look at other examples: only recently Twitter started to be popular among teenagers. (At least in the countries I've lived in in Europe: 1 or 2 years ago no teenager would use it, I now see more and more teenagers on it.) Foursquare is still the domain of 20-somethings, just like Gowalla, or Reddit, which potentially could be interesting for 15/16/17-year olds as well.

How come? Is it because teenagers are not really interested in new social networks, they just want to be where their friends are? Or is it because social networks aren't pitched at them? (Because there's not much money to find maybe?) I'd be interested to hear what other people think about it.