What are the true motivations of programs such as PRISM? According to Snowden they include “diplomatic manipulation, economic espionage, and social control.”

I’m going with the last one being the most important here.

It's absolutely about social control, IMO. Michel Foucault, the influential philosopher-theorist with whom I have some disagreements but not on this subject, wrote a fascinating analysis of the place of surveillance in the construction of social power that I think really nails it. Here's a brief description of his work on the topic, and here's an excerpt from the study itself, Discipline and Punish: the Birth of the Prison. His description of his sometime-metaphor for surveillance by the powerful in society, the Panopticon prison model, has always made sense to me in this era of runaway NSA surveillance:

Hence the major effect of the Panopticon: to induce in the inmate a state of conscious and permanent visibility that assures the automatic functioning of power.

To Foucault surveillance is not really about whether you may do something to subvert social control, it's about the social control that comes from your knowledge that the powerful are watching you. He believed this awareness reinforced social control by ensuring you would never forget who holds power, and why their power is beyond your reach. One way to read his analysis is that hiding the fact of surveillance undermines its fundamental role in controlling society and concentrate power, though hiding the methods of surveillance may not. In this way of thinking, all our outrage and attempts to expose and bring down the NSA/Panopticon can be seen as actually reinforcing its power over us. His prescriptions for dealing with this situation are more complex, controversial, and (I think) difficult to support, but if anyone is still awake after reading this and interested in learning more, there's a ton of writing about it on the interwebs.