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.
If I understand this correctly, I see several flaws:
1. A user can increase his Merit score by deleting old posts (which reduces his Activity)
2. Users without airdropped Merit would start losing their earned Merit because it's not limited to AIRDROPPED_MERIT
3. Users who run out of airdropped Merit will also start losing their earned MeritUpdate:
DdmrDdmr is right, I was wrong!
I've suggested before to link decaying airdropped Merit to post count, not activity. This prevents an old user with more posts than Activity from losing his rank when he returns to the forum and makes just 1 post (and his activity goes up by 14).