Yes, we do. We know ALL of the FBI's high profile terrorism cases. Apart from the Tsarnaev brothers, it has been all cases where one or more fools got snared by a provocateur.
So unless you believe that there is a secret system of arrest, a secret court that hears terrorism trials, and secret prisons all operating inside the US, we know exactly how effective mass surveillance of the US population has been in preventing terrorism.
We already know there are secret courts that hear secret arguments that can't be refuted by defendants/targets. Oh, and if that court involved you, you're legally not even allowed to tell anyone.
We already know that the NSA/FBI/CIA/DEA uses parallel construction to actually prosecute crimes against US citizens.
How do we know this stuff? Someone with clearance risked his life, and gave up his freedom to tell the world. Oh, and the US said they MIGHT NOT waterboard him if he ever came back to the US to face his crimes.
And out of the thousands of documents he had, how many do we have access to? A few handful?
Sure...we know everything </sarcasm (as if you couldn't tell)>