Much of the discussion about passive monitoring has centered on 'paid for' social media monitoring tools. Whilst these tools are very interesting, researchers should not overlook the free and powerful Google Insights [http://www.google.com/insights/search/].
Google Insights allows the user to chart the search terms used by people using Google since 2004. Terms entered into Google's search engine is a great proxy for interest in and this allows the user to investigate interest in brands, words, and phrases over-time, by location. For example, the chart below shows the trend lines for people in the USA typing "American Idol" into Google during 2008, 2009, 2010, and 2011.
I have written a description of Google Insights and how to use it for market research, which you can find here on Vision Critical University.