What is a realistic mathematical function to model link decay in a social network?
Here's a nice paper that explores the question in depth: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4626528/. The authors measure the emotional closeness as frequency of contact and number of different activities done together for a group of 25 people over the period of 18 months. As far as I can tell from the charts, the decay curve is closer to an exponent than to anything else for friends. However, for the kin the emotional closeness increases over time - though, again, in an exponential way.