A few sources, including toolbars like the one mentioned in the article (and including Compete's own toolbar), an opt-in panel (see consumerinput.com, for examp

A few sources, including toolbars like the one mentioned in the article (and including Compete's own toolbar), an opt-in panel (see consumerinput.com, for example), and anonymized browsing data purchased directly from ISPs. Majority of their panel doesn't realize they're sending the data - they've just agreed to do it in a terms of service somewhere.