Activity points are kept for 14 days. This keeps the activity levels based on recent activity, so new members can join and start competing with older members. You can view your points broken down by day here.

It may seem frustrating at first, but it only makes sense. If you are #1 and surfing 100 pages a day for a year, but one day someone else starts surfing 150 pages a day, then eventually they should be ranking higher than you, right?

You may also notice that on some days surfing more, you actually go down in rank! The rankings are based on surfing across the board of all members, so if activity increases from all members, then it's possible that some members passed you in rankings. The opposite is also possible, you could surf less and gain in ranking!