They are part of the World IPv6 Launch measurements found at: http://www.worldipv6launch.org/measurements/

Network providers sign up to join the measurement effort and then Google, Facebook, Akamai and Yahoo measure the connections coming from those networks over IPv6 or IPv4. A person at the Internet Society then aggregates the data from the providers and makes it available on the World IPv6 Launch measurements page. The specific mechanisms used to measure IPv6 by the 4 companies are described at the bottom of that measurements web page.