(As I mentioned, I am reviewing a couple of online tools to track the popularity of ideas over time. Please forgive this segway away from voting reform issues.)
Google Trends is a service by google that allows you to compare the popularity of keywords over time.
In their words:
With Google Trends, you can compare the world’s interest in your favorite topics. Enter up to five topics and see how often they’ve been searched on Google over time. Google Trends also shows how frequently your topics have appeared in Google News stories, and in which geographic regions people have searched for them most.