There's been a fair amount of adult dismay at how young people disclose personal, even intimate information online. In the book, I suggest there are several reasons for this. First, adolescence is a time of experimentation with identity and relationships, and not only is the internet admirably well suited to this but the offline environment is increasingly restrictive, with supervising teachers and worried parents constantly looking over their shoulders.
Second, some of this disclosure is inadvertent - despite their pleasure in social networking, for instance, I found teenagers to struggle with the intricacies of privacy settings, partly because they are fearful of getting it wrong and partly because they are clumsily designed and ill-explained, with categories (e.g. top friends, everyone) that don't match the subtlety of youthful friendship categories.
Third, adults are dismayed because they don't share the same sensibilities as young people. I haven't interviewed anyone who doesn't care who knows what about them, but I've interviewed many who think no-one will be interested and so they worry less about what they post, or who take care over what parents or friends can see but are not interested in the responses of perfect strangers.
In other words, young people are operating with some slightly different conceptions of privacy, but certainly they want control over who knows what about them; it's just that they don't wish to hide everything, they can't always figure out how to reveal what to whom, and anyway they wish to experiment and take a few risks.