I have to say that your article was, by far, the most interesting article that I have read about social networks and how its members can, in a sense, control their privacy. Many members often argue about the "lack of privacy" on social networks; little do they know, they are in control of their own privacy. Of course, people enjoy the attention that they get from other on social networks, which happens to be the sole reason why people fabricate stories to make themselves appear "happier, wealthier, better" than they really are. On the contrary, people want this attention, but not from everyone. They only want certain people to be able to access this information and this is where maintaining agency and power comes in. I have witnessed a girl block her family members from viewing her wall because she knew that she would be posting inappropriate things on her wall. I have also witnessed young girls and boys use jargon when leaving comments and updating their status, so that adults would be oblivious to their conversations. You just have to learn how to manipulate the situation and control your information.
But... you cannot always control the situation.
I once had someone save my picture to their Twitter account, edit it to look silly and upload it to their pictures. I was so upset that Twitter could allow someone to save a picture from someone else's account. I blamed it on their "lack of privacy". I then contacted Twitter to delete the picture for me. I felt like I couldn't control the situation and that I had no power. Even by making my account private, anyone can click on my display picture and save it. The only way to stop someone is by blocking their account and they can always access your account/picture by using someone else's account. You just never know someone else's moves. Like you stated, the critical questions are: who's watching and why are they watching? In a situation like this, it seems like you could never know if someone has saved your picture to their computer or why they saved it. It's just scary to know that someone could possibly pretend to be you online, stare at your picture and try to find you one day or just have your picture saved for another reason. You may never know. So yes, in certain situations, people are able to maintain agency and power -- but when that seems impossible, what is left to do?