knowledge creators and engagement

I am interested in how knowledge building communities can enhance engagement (social and cognitive). But the question is how do you measure engagement? A recent literature review has prompted more questions that answers.

What does engagement look like?
Is it the number of times that students contribute to the discussion? The depth of the contribution? The amount of times an idea is revisited and revised? Or is it something altogether more holistic, such as a reflective thinking portfolio? (Lee, Chan and Aalst, 2006)

Knowledge building teachers, if we think knowledge building enhances engagement in classes, how can identify it.
I would love to hear your ideas and experiences.