The _qca cookie may use your “computer’s IP address, pixel code, referring HTTP location, current HTTP location, search string, time of the access, browser’s time, any searches made on the applicable website, and other statistics” in order to “analyze Log Data from different websites and combine it with other non Personally Identifiable Information to produce the Reports that are made available on the Quantcast.com Site, to enable web publishers and advertisers to deliver audience segments that are appropriate for their products or services.”
According to Edinburgh University __qca expires 26 years after your last visit to a page containing Disqus.
Tim Barlow of attacat says… It is often set by a third party such as a provider of sharing buttons who use it to measure their “audience reach”. Who knows what else it is used for!
I do know that WordPress has a relationship with quantcast – one that it would be nice to see go IMHO