Go read this great article by Gini Dietrich at Spin Sucks: "How to Measure PR: Use These Tools." It includes an inspirational little case study of a content marketing program with the goal of getting people to a free trial. The nice point she makes is that once you settle on what to measure:
"...we know exactly how many people we need to get to the free trial landing page – through content – in order to get them to take it and then what kinds of content – through email – we need to send them so they’ll convert to a customer. It becomes science and math, combined with art, and it works really, really well."
Go read read it right now.