This is a request for pointers to research. I am looking for work on what the phrase "used to know" describes.

Kathryn Moll, one of my students, gave an interesting presentation in class yesterday on the subject of longevity of relationships in social networks, and one of the ramifications of the presentation, obvious in retrospect but surprising to me, was that some time in your teens or twenties, the number of people you know becomes smaller than the number of people you used to know. Put another way, the majority of the people you have met in the last year and know now will be people that, in a year or two and for various reasons, you used to know.

Prior to the internet, maintaining social relationships over geographic distance was time-consuming (letters), expensive (long-distance phonecalls at usurious rates) or both (air travel pre deregulation.) Now it is much less so, but as so often, when we remove an obstacle from our technological networks, we heighten it in our social networks.

If you're reading this page, its pretty good odds that everyone you've met in the last couple of years has email, and if you knew them then, you could still know them now. But you don't. No one does. We require decay in most of our relationships.

I haven't been able to characterize the problem well enough to find anything in citeseer, but if anyone has any pointers to work on how and why a relationship passes from "someone I know" to "someone I used to know", especially when there are no external or technological barriers involved, please send me a pointer, either in the comments section or by mail to claySPAMshirky.com