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device that can track every phone using Wi-Fi within a given area. Just plug this device into a nearby walloutlet to monitor that action in real time. Because we know that more than 60 percent of the U.S. population now owns a smartphone, a couple of months’ worth of data will tell you how many people are likely to be in any area that you’re surveilling on any given day and time of the week. Leave the device plugged in for a few decades and you’ll have a
reasonable
estimate for how many people will be at a specific place, at a specific time, on a specific day of the year. This is the sort of information that big phone companies like Verizon and AT&T have at their fingertips. When you walk around with your cell phone on, you give these companies data about your location. AT&T and Verizon then strip that data of identifying information and sell it to city planners, commercial interests, and others. Verizon even claims the ability to build a demographic profile of people gathered together in a specific place for a specific thing, such as in a stadium for a rock concert or a sporting event. 11
    Navizon puts the same sort of capability in the hands of individuals with small budgets but larger time horizons. Navizon CEO and founder Cyril Houri is marketing the device as a way for entrepreneurs to do location planning. There are some limitations. Because the device measures Wi-Fi from smartphones, it’s also biased toward younger adults (18–25) who are—not surprisingly—more likely to own a smartphone than are people over age sixty-five. High-income earners also show up more often than low-income earners. But the current profile of smartphone users is not the
future
profile.
    Navizon’s analytics system won’t disclose the names of specific people whom the device picks (unless those people opt in to the Navizon buddy network) but the system can recognize individual phones. It has to, in order to count them. If someone follows roughly the same pattern every day, hitting work, the store (or the bar), then home in the same time window, the difference between tagging the phone and tagging the person effectively disappears. MIT researchers Yves-Alexandre de Montjoye, César A. Hidalgo, Michel Verleysen, and Vincent Blondel from the Université catholique de Louvain took a big data set of anonymized GPS andcell phone records for 1.5 million people, the sort of stripped-down location data that Verizon and AT&T sell to corporate partners to figure out the types of people who can be found at specific locations at particular times of day. The data consisted of records of particular phones checking in with particular cell antennas. What the researchers found was that for 95 percent of the subjects, just four location data points were enough to link the mobile data to a unique person. 12
    A growing percentage of smartphone users voluntarily surrender data about themselves wherever they use geo-social apps. Facebook, Twitter, and Google+ all have “check in” features that broadcast your location to people in your network. Other, more creative services will facilitate specific interactions based on what you’re looking to do wherever you happen to be.
    An app called Sonar will identify the VIPs in the room; Banjo will tell you the names of nearby Twitter, Facebook, and Instagram users; a service called Grindr, launched back in 2009, will pinpoint the location of the nearest gay man who may be interested in a relationship—of either the long- or short-term variety.
    To the smartphone-suspicious, these services seem to be more trouble than they’re worth. What’s the value of knowing the Twitter handle of the person at the next table in a restaurant, when, at best, such an app just detracts from the authentic experience of real life? At worst, it’s giving away personal info to strangers.
    However, to a growing number of smartphone owners, check-ins and geo-social Web apps