After news sources revealed that the U.S. government was obtaining phone and Internet data on ordinary Americans, journalist Dan Sinker made an ironic observation: Everyone is up in arms about PRISM, the National Security Agency program that allegedly gave the government access to several Internet companies’ servers.

But when you go to a news site to read about PRISM — including — you’re watched by dozens of commercial data trackers, mostly marketing or analytics firms and advertising networks.

“When I go to the Washington Post to learn about gov data tracking, I’m hit by *fifty* commercial data trackers,” Sinker said Thursday on Twitter. That’s according to a Mozilla Firefox plug-in called Collusion, which claims to find and display third-party trackers as you browse the web.

The hubbub over the government’s secret data program has renewed the scrutiny of all forms of data mining, both the government and private varieties. Programs like PRISM ostensibly target potential terrorists. But commercial data mining has a much broader mandate: Sell stuff to people by targeting ads based on where they go and what they do online. As a result, consumers are increasingly watched by third parties no matter where they browse the Web.

When privacy advocates discuss commercial data tracking, they’re usually referring to a wide range of activities — from data brokers like Acxiom, which aggregates online and offline shopping habits, to services like the Google display network, which tracks browsing data to serve up relevant ads.

There are significant and important differences between commercial and government tracking. Chief among them is that companies are subject to market pressures, while the government is not. Still, many users are unaware of the breadth of tracking by commercial data miners, another anxiety for privacy advocates who see big data eroding consumer rights online. According to the Associated Press, third-party data is a $2 billion industry within the United States alone.

“There is a giant chasm between the type of tracking that companies are engaged in on the web and what people know or think is occurring,” reads a warning on the Web site of the nonprofit Electronic Privacy Information Center. “The general public has very little idea that every second they are on the Internet, their behavior is being tracked.”

The government has been trying to regulate data brokers and other types of data tracking. In March 2012, the Federal Trade Commission urged Congress to pass a law on data-broker transparency, mandating that companies share what consumer information they buy and sell and that they give consumers a “do not track” option. In December, the FTC ordered nine data brokers to explain their practices, and in May the agency warned nine other companies their collection practices could be illegal.

So, what do these data brokers know about you? I reached out to the nine data brokers named in an FTC inquiry late last year and asked to see their profile of my online activities. The results were not surprising: The profiles had compiled my name, age and birthday, my last four addresses, the names of my parents and brother, the value and square footage of my parents’ house and the fact that I buy books and clothes online. Other apps and programs that consumers opt into — such as Amazon and Facebook — presumably have collected more extensive data on their users’ browsing history, shopping habits and demographics. An Austrian man who asked Facebook in 2011 to provide him with the personal data the company had compiled from his account was given a 1,222-page file filled with about his life.

For privacy advocates, the scope of such data-gathering. But for the industry, it’s business as usual. As Jeanette Fitzgerald, the general counsel for the data broker Epsilon, said in December, consumer data tracking can only go so far.

“If it were really that sinister, it wouldn’t work,” she argued. “Consumers would opt out of e-mails. They’d stop going to Web sites. Marketers want to reach their customers. Frankly, there’s no sense offending them.”

When it comes to the government, however, those market forces don’t apply.

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