I would say that specific statements by multiple whistleblowers constitutes evidence, if not proof.
It doesn't matter anyway, the burden is not on citizens to prove that the government is spying on them, the burden is on the government to prove to the citizens that it's no longer lying to the society.
As are you.
How many times are you willing to allow someone, or some agency, to repeatedly lie to you, before you decide to become skeptical?
p(The NSA is lying|we've already witnessed them lying to Congress)?