Wikipedia describes web analytics as “the measurement, collection, analysis and reporting of web data for purposes of understanding and optimizing web usage.” Both trusted and untrusted websites, and whether they are paid or free, covertly collect and analyze user data and allow other “partners” to do the same in exchange for fees.
These partners include advertisers (target advertisements), data brokers (buy and sell user data) and content providers (3rd party content). Each person has a Unique Identification number or UID. This UID is used to collate data collected by various partners in an ecosystem (ISP, websites, data brokers, ad brokers, etc.) into dossiers on each user. This data can then be used to target advertisements, perform background searches by universities, recruiting companies, insurance companies, etc. It is available to anybody willing to pay money for it.