The following information is obsolete.
Quantcast was removed from Jetpack in version 2.8, on January 31, 2014.
The Jetpack plugin includes a module called stats which does quite a nice job of displaying At-A-Glance site statistics in your WordPress Admin pages.
I wasted quite a lot of time trying to track down the code that caused this plugin to be loaded
It wasn’t until I decided to look at the Google Chrome Developer tools network log that I discovered the sequence of events that cause the cookie to be loaded.
What does this mean to you?
Well, if you use the jetpack plugin then you should list __qca as one of the cookies that your site will use.
So, for anyone use the cookie-cat plugin I need to re-generate the cc_mapping.xml file and make it available.
The tracking is normally only performed for users who are NOT logged in, but there is an option to control this so that logged in user’s page views are also recorded. Either way, unless you also have the manual-control plugin, if you’ve got Jetpack then you’ll have __qca too.