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As @paulag said, part of the activity is posting and commenting.
You might be interest in my stats and the related definition of an "inactive" account

Yes, posting and commenting is of importance. I was just trying to find the churn similar to Facebook's calculation based on likes on a quarterly basis. I have already read your informative post. Upvote is the major incentive here as posts made by people like me disappear into oblivion if they do not get good upvote. So I focused only on the voting part.