It's likely that the massive surveillance infrastructure isn't for looking for certain words but to link individuals together and build graphs to explore relationships. During a wiretapped call that investigators are listening in on, a name in dropped. That name could then be run through the NSA's databases to produce not just how the target knows the new person but who that new person knows. There could be an entire terror cell operating totally unknown but unless something links one of the members to terrorism, all of those guys could just be a bowling club for all anyone knows.