The way these guys gather this data is by convincing you to install a toolbar
or similar technology into your browser and you become a ratings generator
sort of like having a box from AC Nielsen sitting on your TV,
except this one is built into your browser.
The difference is that Alexa and Compete's models are all software
and are installed by more people, don't cost much to generate data
and have to grade a million TV stations (websites) instead of 500.