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Twitter, for one: http://www.theregister.co.uk/2012/05/18/twitter_signs_do_not...

The key part is not the websites but the services (ads, analytics, etc) that sites use.

For those, quoting from http://donottrack.us/implementations :

3PMobile, AdInsight, AdOcean, AdTruth, AP News Registry, Blue Cava, BlueKai, BrightTag, Chitika, Effective Measure, eXelate, Jumptap, m6d, Mochi Media, TagMan, Tealium, TruEffect, Ensighten, Twitter.

Note absence of Google with their advertiser hat on, as well as Facebook.

Well, websites are the ones to lose with this, not browsers(except ones with conflict of interest like Google and Microsoft). So websites can say "Hey, a popular browser like Chrome doesn't support it, so why should we?". That's why it's important for all browsers to support it so that there will be more pressure on websites to support it.