To know whether or not PR activities are making an impact, and to know if people are at least reading your hard work, it's a good idea to monitor activities using Google Analytics. Just as bloggers monitor their statistics, it's important to do this with client and organizational websites as well.
For instance, one client I was working with decided to change their company name because they realized that visitors were spending an average of 8 seconds on their blog after the majority migrated from search engines. They didn't find what they were looking for so readers left. This got the organization thinking that they should rebrand in order to boost interest in the company and attract people to their website for the right reasons. They did that and within five months after the rebrand, their readership improved significantly with 47% of unique visitors spending 5 minutes or more on their site- evidence that they were clearly reading content. Not a bad improvement!
It's also a great way to make a case with senior decision-makers on what's working and what's not and where to focus web dollars if there are multiple sites to manage. You can also access valuable data on how well the site is doing with referring sites, and build on ways to secure cross-promotional references if you feel you need to improve in that area.
According to Wikipedia, Google Analytics is the most widely used website statistics service with 57% of the 10,000 most popular websites using it.
Another great tool is Technorati Authority Rating which ranks blog popularity based on its influence in the blogosphere by evaluating linking behaviour.
Food for Thought: What is the most effective online measurement tool you've come across?