A good article I recommend you to read about tracking and measuring PR metrics. Interesting to observe their point of view on this question 🙂

PR is notoriously difficult to quantify. Part of the challenge is
developing PR metrics that will be meaningful over time. If PR
objectives are constantly shifting, certain programs will undoubtedly
get more attention than others. Setting PR metrics is a
semi-existential undertaking. Will things that matter today still
matter a year from now? Two years? Ten? Fifty?

To read the rest of the article : http://icouldntfindanypaper.blogspot.com/2008/09/mozilla-pr-metrics.html
The article is written by Melissa Shapiro, the PR manager for Mozilla.