"Online advertising companies, for instance, can collect huge amounts of information about a person's browsing history, which can reveal "a very comprehensive dossier of what you've been up to" on the Internet, according to Jonathan Mayer, a lawyer and computer scientist affiliated with Stanford's Center for Internet and Society -- not all of which might match up with the persona you present in real life.
Some sort of legal process is generally needed to access the non-public of these data troves, Mayer said -- so there's a limit to how much they can be used to force honesty at the moment. "If the price of invalidating a lie is starting a lawsuit -- I think it's safe to say we won't be invalidating a lot of lies," he said.
But there are less than legal ways to get around that: "One of the most common fact patterns in federal hacking cases is folks with some sort of relationship -- be it business, or other personal -- stealing another person's password," said Mayer."