We use a unique methodology for collecting this data. We collect data from the browsers of site visitors to our exclusive on demand network of small to medium enterprise live stats customers. The sample size for these sites is more than 40,000 urls. The information published is an aggregate of the data from this network of hosted website statistics. The site unique visitor and referral information is summarized on a monthly basis. "
From the site:
"About Our Market Share Statistics
We use a unique methodology for collecting this data. We collect data from the browsers of site visitors to our exclusive on demand network of small to medium enterprise live stats customers. The sample size for these sites is more than 40,000 urls. The information published is an aggregate of the data from this network of hosted website statistics. The site unique visitor and referral information is summarized on a monthly basis. "