I was reading this article in the New York Times, Facebook in a Crowd.
It got me thinking that the concept of polite social attrition doesn't
exist in online social networks. No-one has thought to build them in.

Friends say goodbye or drift apart in real life, why not online?
Polite social attrition functionality would
simply be a means by which to set your social networking account to
dismiss connections to friends after, say, a period of non-personal
contact with them. So, for instance, if you don't have a one-on-one
exchange with a particular person for a period of time, Facebook might
first attempt to nudge both of you, or it could just simply break your
online association automatically and notify after the fact. I suppose
that behavior would be user configurable. The configuration schema
would also allow the setting to operate selectively among parties so
if/when it were deemed acceptable, some people could stay passive
friends forever.
The major advantage would be that
relationships could naturally dissolve just like they do in real life,
without one or both parties making the potentially rude conscious
decision to terminate. It would go far to make sure that friends and
associations are optly relevant for the current life theater of each
party, and of course, help with simple friend list maintenance. For
very social indviduals, it must take hours panning a list of friends in
order to trim the fat.
If social network sites already have this
functionality (I haven't seen it), great. If not, I think it's
something that would be very valuable.