Unlike monitoring, measurement is more concerned with metrics over a specified time period. Whereas monitoring answers the question “Who is talking about [insert keyword] right now and what are they saying?” monitoring answers questions of “How did my keywords perform over time?” “How does that compare to my competitors?” and “What are some trends I can glean to make my product more usable by these people who are giving me feedback?
Great list of Social Media monitoring tools http://wiki.kenburbary.com/. However, we are still very far from having a similarly accurate Social Media measurement toolset. This in my opinion is going to be the huge factor in the real growth towards the accurate commercialization of Social Media.