This is the hear of it, isn't it? You have a whole bunch of people who don't care if Google uses automated scanning of their emails. You expect them to care if the NSA does it? Sure, there is some subset of people who will draw a distinction between the government and a private company, a distinction which isn't baseless, but most just don't care.
Also, look at it from the governments point of view. If you're NSA, do you really feel like you're violating intimate privacies to go through information people knowingly expose to one time acquantances on Facebook? The NSA is made of people, remember. How people view their own information enforces their understanding of where the line is.