When it comes to softening up shoppers and making them more comfortable sharing personal information with retailers, nothing has done a better job than social media sites.

When it comes to softening up shoppers and making them more comfortable sharing personal information with retailers, nothing has done a better job than social media sites. Mobile devices, with their geolocation capabilities and beaming out a continual “this is who I am” signal to anyone who chooses to listen, come in a close second.

But a new study from Carnegie Mellon University changes that perception sharply, in several ways. First, it seems that, over the years since social media giant Facebook first became prominent, the amount of information that people have been sharing publicly has sharply declined, which contradicts what most retail marketing executives have argued. Secondly, when those same consumers believe that the network involved is private, their actions reverse and they are comfortable sharing a lot more data than initially.

Understanding the nuances of how shoppers perceive privacy, what information they consider to be private and what incentives will work to make them give up data is crucial, whether it’s for a CRM (individual) strategy or to merely better understand shoppers in aggregate.

With teen shoppers, the challenge is different. Tactics to get them to surrender a piece of private data has to start with understanding what they consider private. What Facebook and Youtube have indeed done is to make them think many of the most intimate details of their lives are not private. Hence, if you want their E-mail address, a one-time $5-off coupon may be all that’s needed.

For example, one of the earliest—and continuing—questions on Facebook is relationship status. For earlier generations, that question would seem to be the epitome of private information. And yet, Facebookers see it as a central part of their public profile, often as part of the hunt for a new relationship. That means that “are you seeing someone?” is not often considered not private and not especially intrusive. My, how times have changed.

And if you’re trying to sell gift cards or intimate clothing for the opposite gender, such information is quite helpful.

Zip Code—a U.S. postal designation indicating immediate neighborhood—is another item that is moving out of the privacy realm. The California Supreme Court recently ruled that although physical store associates cannot ask that question due to privacy concerns, the online counterparts for those same chains are explicitly permitted to ask. For E-Commerce operations, the question isn’t an evasion of privacy as much as security issue. (But not so for stores? Go figure.)

Privacy cuts both ways. Nordstrom is in the middle of an extensive mobile trial to count shoppers. Nordstrom has been very strict about not seeking any information beyond that which is needed to count bodies and to spot repeat shoppers. But the mobile vendor they are using? Not so much. They’ve promised to be good, but just for the moment. The future is up for grabs.

The CMU study’s focus on areas that consumers consider private—and the fact that consumers will reveal far more in such forums—is controversial. That’s because Facebook’s definition of a private area is hardly private. . Friends lists on Facebook can often move into the hundreds and many lists easily top 1,000. (Full disclosure: Please don’t ask me how many of my LinkedIn connections are people I actually know. It’s a touchy subject.)

And those increasingly large amounts of disclosed private data are not merely shared with that battalion-sized group of intimates. That data also is shared with, as the report notes, “third-party apps, advertisers and Facebook itself.” And there’s no practical limit on who Facebook will choose to share it with. Contractors? Partners? Random winos?

Evan Schuman is editor of StorefrontBacktalk.com, a US-based site that tracks retail technology, e-commerce and mobile commerce issues and a Retail Week content partner. He can be reached on eschuman@storefrontbacktalk.com.