But because big online firms like Google, Yahoo and Facebook also run ad exchanges that place ads across the wider Web, DNT might apply to them too.
The whole thing is a mess. Yahoo recently said it would no longer honor the DNT signal, citing the lack of "a single standard that is effective, easy to use and has been adopted by the broader tech industry." Instead, Yahoo says its users can manage their privacy settings themselves with tools on its site.
It's hard to get a firm count on how many companies honor, in some way, DNT. DoNotTrack.Us, a website maintained by Stanford researcher Jonathan Mayer and Princeton's Narayanan, pegs the number at under two dozen, though the list is not regularly updated.
But maybe the list should be empty. "There is no such thing as Do Not Track right now," said Mike Zaneis, executive vice president, public policy, and general counsel at the Interactive Advertising Bureau. "It's a gimmicky marketing term," he said.
Money could also be driving companies' refusal to honor DNT, given that their businesses largely run on ad dollars. Delivering ads to the right people at the right time is harder if they're hiding from you.
"DNT isn't being honored because advertising companies like Yahoo just don't care very much about user privacy, or haven't been forced to care," said Peter Eckersley, technology projects director at the Electronic Frontier Foundation, via email. Eckersley is also someone who has worked on the DNT technology to develop a standard, so far to no avail.
There are plenty of other privacy-themed browser extensions, search engines and even social networks out there now, designed to block tracking and targeted ads. The browser extensions, like Ghostery or AdBlock Plus, are designed to automatically prevent the person's browser from connecting to ad companies' servers.
Research conducted by Evidon, which makes Ghostery, shows these tools are better at keeping users anonymous than DNT. They work better because using them is like locking your house, versus putting a sign in your yard that says, "keep out," said Princeton's Narayanan.
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