Users of social media are constantly warned to watch what they reveal online. You never know who might read your postings, the argument goes — your grandmother, your boss, potential employers.

To that list of potential readers, you'll have to add the police, at least if you live in Britain.

Wired UK reports that London's Metropolitan Police, or Scotland Yard in popular parlance, has admitted the existence of a team dedicated to monitoring the social media postings of some 9,000 people for signs of political unrest.

The unit has 17 officers and uses what it calls Social Media Intelligence, or SocMint, to scan Facebook, Twitter, YouTube and other social media services 24 hours a day, Wired UK said, adding that the team is developing special tools to smooth the process.

As part of the Metropolitan Police, the unit has jurisdiction in all of England and Wales, and some jurisdiction in the legally distinct "countries" of Scotland and Northern Ireland.

At a security conference in Australia last month, a Scotland Yard official, Umut Ertogral, spoke freely during what he thought was a closed-door meeting.

Social media "almost acts like CCTV [closed-circuit television] on the ground for us, really," Ertogral said, according to the Australian Financial Review.

"If you have something that you don't want anyone to know," Google CEO Eric Schmidt said in 2009, "maybe you shouldn't be doing it in the first place."

This line was creepy enough coming from one of President Obama's confidants and fundraisers. It takes on added weight now that the Washington Post and the Guardian have reported that the National Security Agency's Prism program, in the days before Obama was sworn in, tapped into Google's servers, gaining access to every message sent or received over Gmail.

Google spokesmen, like spokesmen from all the tech companies, deny participating in any such program. So Americans are left to wonder: Was this corporate-government collusion? Was this federal hacking or infiltration of corporate servers?

These are murky questions, so let's return for the time being to Schmidt's point, and the question it raises: If you're upset about the government reading your emails, or knowing whom you call -- when, from where, and for how long -- then what are you hiding?

In other words, why should law-abiding citizens mind federal surveillance?

The answer begins with this distressing reality: None of us scrupulously obeys the law. Technically speaking, we're all criminals.

Federal and state criminal statutes have multiplied like rabbits over the decades, and so now everyone breaks the law, probably every day.

Copy a song to your laptop from a friend's Beyonce CD? You just violated the Digital Millennium Copyright Act. Did you buy some clothes in Delaware because they were tax free? You're probably evading taxes. Did you give your 20-year-old nephew a glass of wine at dinner? Illegal in many states.

Citizens that the federal government wants to indict, the federal government can indict if it monitors them closely enough. That's why it's so disturbing to learn that the federal government doesn't need to obtain a warrant on us in order to get our emails and phone records.

But these surveillance powers are used only for hunting terrorists, Obama says. Even if you take him at his word, because so far there is no evidence to the contrary, think about the capabilities you give to government when it can snoop on your phone records and emails.

Maybe Obama hunts only terrorists with it. But our next president could expand it to follow violent felons -- without having to get specific warrants. Why not drug dealers and sex offenders? Tax evaders come next.

One threat to privacy is Congress expanding the use of these Big Brother tools. Another threat is an administration using it illegally. This happens. President Bush used surveillance powers inappropriately. New York Gov. Eliot Spitzer targeted political opponents with state surveillance.

So what's next? A president targeting hard-core environmentalists or pro-life activists on the suspicion they'll carry out terrorist attacks? This may not sound likely, but recall scare stories about "ecoterrorists," and how Obama's Department of Homeland Security has warned that Tea Partiers are serious threats...