I've developed a deep enthusiasm for the search tool in Google Trends. The tool allows you to enter a term or multiple terms, and builds graphs and lists based on how many searches there were using those keywords on Google. Since Google is the dominant search engine, this will give you a good idea of how much interest there is in the subject you've chosen, and you can compare two or more topics. It also allows you to choose particular geographies, so you can compare how popular, say, Justin Beiber is in the United States vs Argentina.
For the past few days, when I read an article, I use Google Trends to determine how much interest the public has in the subject or subjects of the article. This morning I read the article Big Money: Billionaires at Bat on Politico. It centered around the book Big Money: 2.5 Billion Dollars, One Suspicious Vehicle, and a Pimp on the Trail of the Ultra-Rich Hijacking American Politics by Kenneth P. Vogel. The book is about the increased influence of billionaire donors on American politics.
Needless to say the article mentioned the best known politically active billionaires, the Koch brothers on the conservative side, and George Soros on the progressive end of the spectrum. I did a comparison search using the words "Koch brothers" and "George Soros", and here are two of the charts that it yielded.
If you select the link under the chart entitled "View full report in Google Trends" you can filter the information in various ways, determine how much interest there has been in the Koch brothers and George Soros in different geographical regions, and view the data in a number of different ways. One interesting thing is that while Soros was well known by at least 2004, the Koch brothers only came into the public eye around the time of the 2010 midterm elections.
In summary, Google Trends is a good tool for tracking public interest on any topic which generates enough searches to put together meaningful charts. It will tell you when interest in a topic began, and what direction the public's interest in the subject is headed.