How likely is the NSA PRISM program to catch a terrorist? 
So? 1/10,100 isn't bad at all. Suppose you have many independent tests, each creating watchlists. Suddenly you have the intersection of individuals hitting each of these, and the number of "interesting" individuals narrows. This is not a univariate problem; they're likely correlating everything to everything, and have adaptive snooping methods based on how high you are ranked a risk. Once you're high enough, field offices know about you and will keep a closer eye. It's really not a needle in a haystack (and as a privacy nerd, it pains me to say this, but I bet it is working).