There is simply no way human beings can sort through that much data. That means relying on gadgets and software to do the sorting for the humans. Anyone who manages big data can tell you how corrupt most data sets really are. Names spelled different ways, bits of information incorrectly transcribed, copy errors, format errors, import errors are all low probability events but, when you're dealing with billions of records, there are a lot of them. Just in general, gadget security doesn't work.
In nearly every terrorist event that's happened in the U.S., the FBI had tips from alert citizens. That was true for 9/11 and almost all of them in between. The FBI even interviewed the Boston Marathon bombers. HUMINT works.
Funny that the FBI screw ups don't get more media attention. In nearly every case they didn't effectively use the information they had, so how is more information going to make things better?