The question Boyd raises is similar to a question a foreign visitor may ask from another planet. What's all the hype about. It appears that what she is expressing is communication is occurring on a different level than what we are accustomed to. She mentions the portion of myspace in which one can post comments, others view this communication and learn things about us by observing previous comments made by others. In turn the other individual is influenced by a previous, totally unrelated comment, thereby they may treat you in such a way that is indicative of a previous message.
Where Boyd uses the expression, "people are writing themselves into being," I believe this to mean that individuals are creating their selves that may not surface in the direct confrontational world of outside, away from your computer screen that describes what you are or who you want to be. It seems this will extend into other boundaries of the individuals life, but a reliable question to ask in determining the mind of the person is whether the website is reflective of the individual or the individual is reflective of their website. If the website is the first mover, then it is likely they will become an extension of their online creation.
"Boyd talks about youth culture as against adult publics, is this true? Can children rebel against adults online?'
I think most kids when they are coming into being with their identity and starting to get a sense of the world around them are usually left feeling like they can do better than what is around them. I think if what is around seems lame and pathetic to kids they're going to rebel against it. Maybe this is not always true but in recent decades we have seen, most often a disposal of the ruling cultural "cool" and a replacement of what is deemed hip by the emerging youth culture. Therefore it seems likely that kids will use what ever outlets they can find to rebel against the ruling hegemony of coolness. Myspace will no doubt be a tool for that expression against that class, and their parents, of course. I think rebellion towards parents factor is most often the display of aggression, myspace will allow the kids to act out against their parents morals, beliefs, and values.