In exchange for a digital identity, people have given up their privacy.

Google has kept archives of emails sent and friends’ replies for the last six years. Facebook tracks people’s activities all over the Internet, even when they are logged off. And Twitter taps into entire address books when people use the “find friends” option and archives it for 18 months.

For those who do not use Facebook or Twitter, that does not mean no one is watching them, said Dahlia Lithwick, who reports on the law and the U.S. Supreme Court as a senior editor of Slate magazine. People can be tracked by turning on their phones, using an E-ZPass or using a Starbucks card.