I also puzzled by this matter just yesterday. I also newbie, answer is below, get activity up to post topic and online time.

here is activity calculate rule:

How is forum activity calculated?
Activity is calculated with the following formula:
Code:
activity = min(time * 14, posts)

Activity is increased by staying logged in or by posting and is updated every 30 minutes. There is a maximum of 14 activity points every 2 weeks.