How do you calculate activity rank?
The activity rank is a measure of how much each member contributes to Critique Circle through stories, critiques and forum posts. This is a very simple indication of a member's overall activity. Someone with rank 1 is the most active member on CC, plain and simple. 
Each type of activity contributes differently to your rank calculation. For example, you will get much more activity points by sending in a full-credit crit than you will by starting a new thread. The exact ratio has been determined through heuristics and is a well kept secret.
As the activity rank is calculated in relation to other members, you can see at a glance how you compare to others. This information is accessible from your dashboard page. You can also view the activity rank of other members from their member pages or by hovering over their name elsewhere on CC.
The activity rank takes into account activity for the last 30 days. Anything that is posted before that time will not contribute to the current activity rank. Therefore, this is a very good indication of just how active a member is right now.
If you do not want others to see your activity rank or other indications of activity (such as number of stories posted or crits given), you can check "Hide Activity Information" in your account settings page.