By Reed Albergotti 

Online retailers and publishers are pushing back against Facebook Inc.'s efforts to track users across the Internet, fearing that the data it vacuums up to target ads will give the social network too much of an edge.

Web traffic experts say there is less data flowing from some sites to Facebook, suggesting they have been reprogrammed to hold back information.

Facebook has long kept track of the websites its users visit when they aren't on the social network. Three months ago, it began using the data to build more detailed user profiles, allowing advertisers to target people with more personalized marketing pitches.

That has rankled some retailers, advertisers and Internet publishers, which worry that the wider use of browsing history will hand Facebook, and potentially their own rivals, more information about existing and prospective customers.

In response, some businesses appear to have changed their sites to send less data to Facebook; others say they are considering such moves.

The change "freaked everyone out and rightly so," says Vivek Vaidya, co-founder and chief technology officer at Krux Digital Inc., which helps large online media companies manage their data. Mr. Vaidya said he has spoken to several media clients and "all of them, to some degree, have expressed concern about this."

The concern underscores the love-hate relationship between website owners and Facebook. Publishers, for instance, like it when their content is shared among Facebook's 1.3 billion users, as they hope to win new readers. And online retailers have had success combining their data with the social network's to track customers online.

But both groups are wary of Facebook's huge store of data. While other websites keep track of individuals who stop by, noting them by their computers, the difference is that Facebook has real names--allowing it to do more with the information that accumulates about a person's browsing and shopping habits. Facebook, meanwhile, has grown into an advertising juggernaut, with expected revenue of $12.2 billion this year, putting it in direct competition with publishers for ad dollars.

Facebook says it hasn't yet begun using data from other sites. When it does, it will let publishers opt out of the system, a spokesman said, though doing so might reduce how much of their content is seen by other Facebook users.

Publishers and advertisers that want users to share content on Facebook install a small bit of Facebook code, called a pixel, on their sites; the pixel is often associated with Facebook's "Like" and "Share" buttons. Many websites have installed the code, allowing the social network to record a significant amount of Internet activity.

Facebook places another bit of code, known as a cookie, on its users' computers. When a user visits other websites that have Facebook's code, Facebook can recognize the cookie, building a record of how the user surfs the Web.

Facebook also can track users on their phones. Some mobile app publishers, aiming to optimize their advertising on the social network, let Facebook know the unique hardware identifiers of their app users. Facebook can match the IDs to those of its own mobile users.

(In order to access Facebook, all users are required to agree to the company's terms of service, which allow for such data to be collected.)

Daniel Cotlar is chief marketing officer for Blinds.com, a large online retailer of window blinds and a Facebook advertiser. He fears that Facebook will identify visitors to Blinds.com as interested in home furnishings, and then help his competitors advertise to them.

Mr. Cotlar said he is considering removing Facebook's trackers from Blinds.com. "We haven't ripped it down yet, but we're talking about it," he said.

A spokesman for Facebook said its profiles of users won't rely too heavily on any one website, such as Blinds.com. The spokesman said advertisers can opt out of allowing their data to be used in ad targeting--but they won't be able to target users based on data gathered from other sites.

Some publishers and retailers appear to have curbed how much data they are sending to Facebook, according to Ghostery, a maker of privacy software. Ghostery's software recognizes Facebook's pixel code.

Ghostery says that across the Web, Facebook's pixels are sending data back to Facebook as often as ever. But Ghostery says that since the spring, it has seen Facebook's code less often on certain well-known sites, including those of the New York Times Co., AirBnb Inc., Williams-Sonoma Inc. and Abercrombie & Fitch Co.

Ghostery executives said those companies appear to have modified the pixels on their sites. Chief Executive Scott Meyer said Ghostery doesn't know why the companies modified the pixels, but they may have wanted to send less data to Facebook.

Those companies declined to comment or didn't respond to requests for comment.

A Facebook spokesman counters that some website operators may have modified the pixels to speed the response of their sites.

John Strabley, director of strategy for Quaero, a firm that helps companies like ESPN and MSNBC manage the customer data they gather online, said many of his clients would rather not share data with Facebook--but they don't want to miss out on the potential traffic from readers who share their content on the social network.

Mr. Strabley thinks sharing data with Facebook might help the social network win advertising dollars that would otherwise go to publishers.

Advertisers that want to reach sports fans, for example, may choose targeted Facebook ads over ads on Sports Illustrated's website.

Moreover, Facebook ads can be more cost-effective because they allow advertisers to target specific demographic groups in narrow geographic locations.

Online publishers are "sharing their entire audience back to Facebook and what are they getting for it?" asks Mr. Strabley. "I can pretty much guarantee that the value publishers are getting from that is not worth it," he said.

Mr. Strabley wouldn't name the clients he said were concerned.

Sports Illustrated, MSNBC and ESPN all declined to comment.

Marketers will spend more on digital ads than either newspapers or magazines this year, according to eMarketer, in part because advertisers can target digital ads to smaller subsections of potential customers.

To be sure, many other companies, including Google Inc., track the browsing histories of Internet users to help target advertising. But advertisers say sending information to Google doesn't scare them as much as sending information to Facebook, mainly because Facebook knows users' real identities.

Google has to infer users' interests from Web browsing, which can be misleading.

A Facebook spokesman said the company's personal data is "better" than Google's, but Google's is similarly detailed.

Google declined to comment.

Write to Reed Albergotti at reed.albergotti@wsj.com

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