'To be remembered after we are dead," wrote Hazlitt, "is but poor recompense for being treated with contempt while we are living." Cue President "George W" Obama in the matter of telephone surveillance by his National Security Agency. The fact that for the past seven years the agency has been collecting details of every telephone call placed in the United States without a warrant was, he intoned, no reason for Americans to be alarmed. "Nobody is listening to your telephone calls," he cooed. The torch was then passed to Dianne Feinstein, chair of the Senate intelligence committee, who was likewise on bromide-dispensing duty. "This is just metadata," she burbled, "there is no content involved."

At which point the thought uppermost in one's mind is: what kind of idiots do they take us for? Of course there's no content involved, for the simple reason that content is a pain in the butt from the point of view of modern surveillance. First, you have to listen to the damned recordings, and that requires people (because even today, computers are not great at understanding everyday conversation) and time. And although Senator Feinstein let slip that the FBI already employs 10,000 people "doing intelligence on counter-terrorism", even that Stasi-scale mob isn't a match for the torrent of voice recordings that Verizon and co could cough up daily for the spooks.

So in this business at least, content isn't king. It's the metadata – the call logs showing who called whom, from which location and for how long – that you want. Why? Because that's the stuff that is machine-readable, and therefore searchable. Imagine, for a moment, that you're an NSA operative in Fort Meade, Maryland. You have a telephone number of someone you regard as potentially "interesting". Type the number into a search box and up comes a list of every handset that has ever called, or been called by, it. After that, it's a matter of seconds before you have a network graph of second-, third- or fourth-degree connections to that original number. Map those on to electronic directories to get names and addresses, obtain a secret authorisation from the Fisa court (which has 11 federal judges so that it can sit round the clock, seven days a week), then dispatch a Prism subpoena to Facebook and co and make some coffee while waiting for the results. Repeat the process with the resulting email contact lists and – bingo! – you have a mass surveillance programme as good as anything Vladimir Putin could put together. And you've never had to sully your hands – or your conscience – with that precious "content" that civil libertarians get so worked up about.

Do people fall for the "it's only metadata" guff? If they do, my hunch is that it's because their intuitions haven't kept pace with the extent that mobile technology has pervaded our lives, or with the scale of the data that outfits such as the NSA have been accumulating. Most people would be discomfited to learn how detailed a reconstruction of their lives their mobile phone operator could produce if required – right down to a pretty good guess at when they have been speeding in their cars. Four years ago a German Green party politician, Malte Spitz, sued to have Deutsche Telekom hand over six months of his phone data that he then made available to Zeit Online. The paper then did what any decent NSA operative would do, namely combine his phone's geolocation data with information relating to his life as a politician – Twitter feeds, blog entries and websites – to create an extraordinary animated reconstruction of a day in his life.

It's this revelatory power that enables metadata to expose far more than what a target is talking about. "Metadata," says Matt Blaze, a crypto researcher at the University of Pennsylvania, "is our context. And that can reveal far more about us – both individually and as groups – than the words we speak. Context yields insights into who we are and the implicit, hidden relationships between us. A complete set of all the calling records for an entire country is therefore a record not just of how the phone is used, but, coupled with powerful software, of our importance to each other, our interests, values, and the various roles we play." So even if the NSA never listened to a single telephone conversation, it would still be able to build what Blaze calls "a national relationship database".

In the old days, the medium was the message. Now it's the metadata.