One Moodle Plugin that thought would be interesting to track forum participation would be the Forum Graph. It appear to generate a report similar to what I see are teachers create when they are tracking the Socratic discussions in their classrooms on paper.
We haven't installed or tried this plugin https://moodle.org/plugins/report_forumgraph yet, so I shouldn't say much. But maybe someone else has used it and would share because we are interested...