I am a "big data" engineer, and I have no idea how they could do something like this. However my inability to theorize a potential method of collecting this data is not evidence for/against its existence. Remember the NSA employs some of the most brilliant software engineers and mathematicians on the planet whose job it is over the course of years to figure this sort of stuff out. I'm sure they've got a few tricks up their sleeve. Time will tell of course, and more revelations are reportedly "imminent"