Net Applications collects data from the browsers of site visitors to their exclusive on-demand network of HitsLink Analytics and SharePost clients. The network includes over 40,000 websites worldwide. Net Applications counts unique visitors to our network sites, and only count one unique visit to each network site per day. The data is compiled from approximately 160 million unique visits per month. The information published is an aggregation of the data from this network of hosted website traffic statistics.
More info vía Net Applications here.