Do social sites with karma need a decay function?
epoweripi: I'm just thinking out loud.Elders a.k.a old people in a normal society are respected because they have been in the 'system' longer, irrespective of  their accomplishments.
That to me is one form of karma (Age)Also, ppl who have accomplished a lot irrespective of age are looked up as role models in the society : the form of karma that most social sites capture - contribution.Similarly bad acts reduce one's karma - independent of the good he has done - Hans Reiser comes to mind.The total karma could be a function of (age, +ve contribution, -ves). 
The age can be based on some condition like page views/logins done etc. - passive contribution OR can be unconditional too (today - creation date).Active contributions can be also be classified into major and minor. Say a particular post generated > 1000+ points. It has to be given importance irrespective of age. Smaller events can be dampened - just like in the real world - no big deal.
Same goes for -ves too.  murder != pickpocket.my view, karma = function (age, major +ve, minor +ve, major -ve, minor -ve)and only the minors need to be decayed.