In the users page there is an option which sorts Meta users by participation. How does the system measure this factor? By number of posts and comments? Or getting more upvotes and less downvotes in Meta posts? or something else?
The measure is the number of posts, comments, edits, and votes cast over the last 60 days.
This is explained in the hover-text.
The official source also mentions that deleted content is not taken into account and that self-edits do not count.
While it is not put on display one can also sort user on main by this criterion: https://math.stackexchange.com/users?tab=participation