First of all, please note that the link you have provided leads to a Google Books preview. The preview says that it has omitted some pages in the selection that you are asking about. Therefore, we do not have access to the whole thing that you are supposed to have read.
With that said, my “take away point” from this is that we must all remember that what we do online (or when we are talking on our cell phones) is really not very private at all. The preview at the link you provide tries to emphasize this. It points out that the things we do online are “public and permanent.” This is something that I think about all the time when making decisions about what I will allow my children to have in the way of technology. It is clear that young people can make very bad decisions about what to take pictures of and what to post. They should realize (as should many adults) that anything posted online can end up becoming very public. This could cause a person a great deal of pain in their lives. This, to me, is the most important point in the passage that is visible to us.