Which audience analysis measurements principles and techniques are most effective, and why?

Measuring audience size for newspaper and magazine audits is very easy to analyze. It is measured by circulation. The data is formed by using pressruns, subscribtion sales and unsold copies returned from newsracks. Radio and television audiences are harder to measure. There is an entire ratings industry to analyze audience data to determine a fair price for advertisements. Nielsen ratings rank every prime-time network show in weekly Neilsen rating. These shows are ranked by audience size.
Personally, I think the most effective audience measurement technique is found over the internet. This would be Facebook and the companies ability to get easily measurable data from their audience, for free. Users pump measurable data into Facebook's database, this allows them to only have advertisements on their page that cater to the individual user. They eliminate wasted space by ensuring that every advertisement on the page is directly applied to the person that is viewing it.