[Wall Street Journal] A study released Tuesday shows that 45% of the top 185 U.S. websites transmit identifying details about their visitors to at least four outside websites.The data transmitted was primarily a “username” – which is the name a person uses to log into a website – or a user ID assigned by the website to a user. It was usually transmitted through referrers – which is information about the web page transmitted automatically.
http://www.huffingtonpost.com/2011/10/11/stanford-privacy-study-tracking-the-trackers_n_1005940.html