"Many people are logged in to Facebook all the time. I would think that an explicit login overrides any DNT header."
As a user, I want to be stay logged in to Facebook when posting a TC comment. But I don't want Facebook to track me when I visit some random site with a like button. How do we draw the line here?
A solution that I've found works well for me is simply disabling cookies, and whitelisting any site that I want to use that requires them, or that I am attempting to log in to.
This is made easier for me by the fact that I don't use Facebook.
I think Facebook and Disqus are different cases. The only reason anyone ever logs into Disqus is to be able to use it on third-party sites. Not so for Facebook. People's intentions matter.
If you don't want to be tracked, the site that handles authentication can still keep you logged in, but it isn't allowed to track your whereabouts.
In the case of facebook they can show the "like" button, but they cant use the information that it has been shown for you on a particular site, on a particular date/time.
Facebook's entire purpose is to show you things you are interested in. The fact you didn't like something on a given site is nearly as useful as if you did.
As part of its functionality it relies on the ability to uniquely identify a user across sites (to provide user authentication).
There are quite a few other services that work in a similar way.