Well first of all Yahoo! has some 550.000.000 unique visitors every month and those visitors are giving their data away by signing and out of various services such as Flickr, del.icio.us, Kelkoo (a shopping search engine which operates in 10 European countries), Yahoo! Answers, Yahoo! Messenger, Yahoo! Mail serving over 260 million users, Yahoo! GeoCities, and Yahoo! Finance, Yahoo! Games, Yahoo! Gallery, Yahoo! Buzz and more. This data is a sample of information collected across the Yahoo Network.
A list of some of the Yahoo! owned sites and services: