How does Quantcast get its information?

[figma12]

I am curious to know how does Quantcast get its information? It seems incredibly weird that they can know so much - from open social profiles perhaps?

Thanks in advance!

[Jake]

Typically these stats services get their core data from things like add-on browser toolbars installed on a small sampling of the population, which collect data on each page a person visits.

Although they say in the below link:
"We collect directly-measured data from the millions of web destinations controlled by our Quantified publishers."

https://www.quantcast.com/learning-cente...ology-faq/