The econsultancy blog has a great post on social interaction measurements. It’s worth a read.
Katie Paine also has another post worth reading (she has a lot of posts worth reading). Here, she talks about problems in calculating the value of PR, based on what the equivalent ad space would cost. Measuring that way is so out-of-date, it always surprises me that it is still used. PR is about relationships, educating people about an issue or cause and helping to inform opinion. Comparing it to what an ad would cost is of no benefit.