Slickstream publishers sometimes ask me why we use "activity time" as one of our key engagement metrics. The answer is easy. It is directly correlated to revenue -- and everyone wants to know how they can increase revenue.
First, let me explain what I mean by "activity time". This is the active time that a viewer spends on your site. Don't confuse this with session duration. That is a metric that you'll see in Google Analytics. But for a variety of reasons, it is a very poor measure of engagement. (Just think about someone who opens one of your pages and then goes out to lunch while leaving that page open in their browser.)