Jane Mayer for The New Yorker:
The gist of the [White House's] defense was that, in contrast to what took place under the Bush Administration, this form of secret domestic surveillance was legitimate because Congress had authorized it, and the judicial branch had ratified it, and the actual words spoken by one American to another were still private. So how bad could it be?
The answer, according to the mathematician and former Sun Microsystems engineer Susan Landau, whom I interviewed while reporting on the plight of the former N.S.A. whistleblower Thomas Drake and who is also the author of “Surveillance or Security?,” is that it’s worse than many might think.
“The public doesn’t understand,” she told me, speaking about so-called metadata. “It’s much more intrusive than content.” She explained that the government can learn immense amounts of proprietary information by studying “who you call, and who they call. If you can track that, you know exactly what is happening—you don’t need the content.”
It looks like even encryption wouldn't protect against this kind of data mining. If you send any direct messages to people, the metadata will show the connection.
In analog terms, it's like someone having access to your phone records and being able to read all the envelopes of your mail, even though they assiduously avoided listening in, or opening any mail. They might not know the content of your calls or letters, but they know who you know, and who knows you, and how often and exactly when you contact them. That can be an awful lot of information.