SPYING: Stanford Researchers: It Is Trivially Easy to Match Metadata to Real People.
On the other hand, there’s this: NSA Struggles to Make Sense of Flood of Surveillance Data: Spy Agency Drowns in Useless Data, Impeding Work, Former Employee Claims.
Downside: They can’t find terrorists by data-trawling, but if they’ve identified someone in advance, like, say, a political enemy, they can easily find dirt on them.