Not long before headlines exposed National Security Agency programs that secretly collect records of Americans' phone calls, another surveillance system got far less attention: Nordstrom, the department store chain, acknowledged it was tracking customers without their knowledge in 17 stores.

Nordstrom had hired a company to log a unique number emitted by shoppers' smartphones, which automatically connected to Wi-Fi systems as they moved through the stores. Shortly after a Dallas TV station broke the story in May, Nordstrom announced it was discontinuing the program.

The company that sold the tracking service, Euclid Analytics, has tracked 50 million devices in 4,000 locations for 100 corporate and other customers, its founder has said. Shoppers are free to opt out, but the process is complex - they must enter their phone's media access control address, known as a MAC address, on Euclid's website.

Self-confessed leaker Edward Snowden's disclosures about domestic spying by the NSA have sparked a broad debate about whether the government is using sophisticated surveillance and data-mining techniques on its own citizens without sufficient oversight.

But information gathered and exploited by Internet giants such as Google, Amazon and Facebook - and traded by lesser-known data brokers such as Datalogix and Acxiom - can be more revealing than what the NSA can legally collect on most Americans. Few consumers understand what data are being shared, with whom, or how the information is being used.

"We normally think of the NSA as being far ahead of corporate America, but I'm not so sure they are that far ahead anymore," said Mark Herschberg, chief technology officer at Madison Logic, a New York-based company that provides data for advertisers.

"There are thousands of companies out there collecting information on customers, and together they are really aggregating quite a bit of data," he added. "Google is reading through your email. Amazon is looking at not just what you buy, but what you shop for."