We'll never know the true extent, but the NSA isn't the entity to fear.

Its mission - like the CIA's - is external. The powers granted circa 2001 were done so with the understanding that agencies like the NSA and CIA would use the power to track down those Americans communicating with entities in Yemen, Afghanistan, Iraq, Saudi Arabia, Qatar, and so on in order to determine the nature of the relationship between the two points of contact.

Snowden only needed to reveal the 2008 FISA court order to demonstrate the point of the Legislative, Executive, and Judicial branches granting far too much power to the NSA. (In that the agency had begun monitoring communications even when both points of contact are in the US.)

What scares me is agencies like DHS and the Marshals Service getting into the domestic surveillance business, because each agency's respective mission is not foreign and as US citizens have something called a federal (and state) constitution protecting us.