I just cancelled my AMEX card and was updating the wiki to remove my referral links, but I would expect that most people would not remove their dead links.
I think it would be useful to timestamp referrals, and delete them after a year to ensure that only active referral codes get used.
It could be as simple as including the date with the referral, or something more complex.
Amex referral link
https://www.ozbargain.com.au/wiki/american_express_referrals
Update 14/6/17: No longer relevant as referral wiki's are no longer allowed. See here.
The wiki is a stop gap solution and we are trying to migrate as many sites to the user referral system which allows me to set an expiry date on the referral data. For example ING Direct's referrals expire after 90 days. However at the moment the referral system is one-per-store which does not work with American Express. Something that we'll eventually fix this year.