I believe that throwing in the towel is appropriate because in the early 1990s I was deeply involved in research into the mathematical foundations of computer security (that's what my doctorate is in) and there was a paper written by a guy called Jeremy Jacob(1) that proved that security and functionality are at odds with each other. Thus my basis for my belief that as Facebook adds functionality security will worsen is the underlying theory.
You may not think of security in this context (calling it privacy) but it's all about the same fundamental thing: the control of information flow. In the privacy context it's information flow about people. Jacob's theorem shows that Facebook's privacy will worsen over time as new functionality is added.
> that proved that security and functionality are at odds with each other. Thus my basis for my belief that as Facebook adds functionality security will worsen is the underlying theory
You are making a huge assumption that Facebook is as secure as it can be at its level of functionality. It is easy to see that they could add a tiny bit of functionality that barely hurts security and fix the "add any of your friends to groups" bug that their security and functionality would increase.
I haven't read the article in full yet. So far it appears that the theorem is explicitly about "preventing unauthorized modification of data." Here we're talking about unauthorized read access, not write access. Can you comment on that?
You may not think of security in this context (calling it privacy) but it's all about the same fundamental thing: the control of information flow. In the privacy context it's information flow about people. Jacob's theorem shows that Facebook's privacy will worsen over time as new functionality is added.
(1) http://www.informatik.uni-trier.de/~ley/db/conf/csfw/csfw199...