Facebook says the program works anonymously, so that neither Facebook nor the advertisers know the individual people who are being matched.
But perfectly targeted ads provided by Facebook's technology eventually may scare more users. The retailer Target generated controversy in 2012 after it was revealed to have used purchasing data and demographic information to identify pregnant women and send them promotional materials geared toward their (unborn) babies.
Or some Facebook users might love seeing just the right ad, on just the right device.
Some privacy experts, when asked to comment on Atlas, said they didn't know enough yet to fully gauge the extent to which it raised new privacy red flags. That didn't stop a German consumer group from protesting it immediately.
For others, their biggest questions revolved around what choices users have, if any, to opt out.
"We've heard a lot about the benefits to advertisers, but not as much about controls for users," said Chris Babel, CEO at TRUSTe, a San Francisco-based company that analyzes and provides services around data privacy.
Unfortunately, opting out of tracking through Atlas, or most other systems, is not easy. You can opt out of ad targeting, which would include Atlas, by visiting the Digital Advertising Alliance opt out page. However, that doesn't change what information is collected.
Facebook this past June started offering more controls to users to help them see why certain ads are shown to them, but the controls don't stop tracking altogether.
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