The common defense of sweeping surveillance is that it serves an important purpose, finding terrorists, and it has succeeded in that purpose.
Ignoring the very serious problems with framing the debate in such a way it's also fundamentally misleading. There have been many cases of "terrorist plots" within the US having been foiled over the last few years, but many if not most of these are not as serious as most would believe. A stereotypical "foiled terrorist plot" begins with a radicalized individual who somehow comes to the attention of the authorities. Then the FBI spins up an elaborate sting to essentially entrap the radicalized individual into committing to some sort of attack, often providing fake bombs in the process. And just before they go through with their planned "attack" the FBI arrests them.
Examples: http://en.wikipedia.org/wiki/Amine_El_Khalifi and http://en.wikipedia.org/wiki/Mohamed_Osman_Mohamud and http://en.wikipedia.org/wiki/Farooque_Ahmed and http://en.wikipedia.org/wiki/Sami_Osmakac
Meanwhile, the FBI, NSA, TSA, et al have failed to foil many much more serious attacks, such as the Boston bombings, the Fort Hood shooting, the "underwear bomber", the failed 2010 Times Square bombing attempt, etc.