I could see decaying airdropped merit, though I'm not convinced of it currently. If it was done, it may be best to do something like this:
- Your current activity score as of is recorded, which I'll call EPOCH_ACTIVITY.
- Your merit score is defined as max(0, AIRDROPPED_MERIT-(CURRENT_ACTIVITY-EPOCH_ACTIVITY)/7) + EARNED_MERIT.
In this case all active users will lose their airdropped merits at a same rate.
The rate of losing airdropped merits can be a function of earned merits in the last X days too.
For example, if someone had received 500 airdropped merits and has also earned 500 merits doesn't lose airdropped merits. Because this user deserved those airdropped merits.