• by Wendy Davis , Staff Writer @wendyndavis, July 30, 2013

Pinterest, like the microblogging service Twitter, has promised to stop collecting data about users' Web-surfing activities if they turn on a do-not-track header.

Pinterest is currently able to glean a great deal of information about users who visit sites that carry a “Pin It” button. The company said in a blog post on Friday that it will keep that data for up to 30 days, and draws on the information in order to show users' personalized material.

But Pinterest apparently recognizes that not everyone appreciates it when publishers decide to “personalize” sites for users based on their Web activity. In some situations, publishers make the wrong inferences, and the personalized material doesn't reflect users' interests. But sometimes, even if publishers make the right inferences, people just don't want their activity tracked.

Pinterest is one of a very small number of Web companies not only to recognize that some people dislike tracking, but also let them decline via their browsers. Many ad networks that let people opt out of tracking require them to do so by clicking a link that sets an opt-out cookie. The problem with that technology is that it's unstable; when people delete their cookies, they end up erasing their opt-out cookies along with the others.

Browser-based signals, by contrast, are easy to set. They're also relatively permanent.

But some ad industry representatives say that browser signals don't necessarily reflect users' opinions. That's because the signals can be turned on by companies as well as users. Microsoft, for instance, is activating do-not-track by default for some users who install the latest version of Internet Explorer. The security software company AVG also turns on do-not-track by default for some users.

In fact, some in the ad industry predict that as many as 50% of Web users could soon be sending the do-not-track signal -- in many cases without expressly turning it on. For that reason, the ad industry says it's not realistic to expect companies to honor the signal.

For its part, Pinterest isn't distinguishing between do-not-track signals that it believes users personally turn on and signals activated by programs that users have chosen to install