An invalid defense for vacuuming metadata

Eric Posner's piece "The NSA's Metadata Program Is Perfectly Constitutional" supports its legal argument (a sound one based on existing Supreme Court precedent) with what at first blush seems to be an appeal to common sense. Posner asserts:

People can more easily find out things about each other today than in 1979, thanks to the Web, and so people now expect strangers—including potential friends, mates, and bosses—to know more about them today than they did in the past. People can also more easily share personal information about themselves, and rather than refrain from doing so in order to protect their privacy, they enthusiastically post photos and videos of themselves on Facebook and other social media sites. Thus, it is possible that people’s sense of privacy is also greatly altered, as if the whole country moved from a big city to a small town, trading in the benefits of anonymity and independence for the advantages of community and security.

It's always risky to proclaim, "Thus it is" and make that a crux of your argument. Even if he's right — and I'm far from convinced he is (my sense of privacy isn't "greatly altered") — his argument is irrelevant. The government can mine our social media to its heart's content: that doesn't give it free rein to poke into other aspects of our lives. (Well, there is that nasty Supreme Court precedent, but I think that rests on shaky reasoning. I hew to Justice Marshall's dissent, and I think a lot of other people do, too.)

Posner is simply too eager to convince us we have moved beyond the expectation of privacy. His eagerness betrays his knowledge that the argument holds no water.