Time Value Decay
noun
The decline of the marketing value derived from actions taken by a marketer for any algorithm based platform the further away from the date that the marketing action first took place.

The time value decay of Jennifer’s Instagram post has made it nearly invisible to her followers a few days later.

Explanation
Algorithms employed by platforms on the internet frequently change in order to increase profits, take advantage of new technologies, and fight spam. Social algorithms frequently change in order to reflect the changes to tastes and preferences of their users, and social algorithms that use cummulative signals such as total number of votes or reviews may reduce the value of older data points over time. This is the time value decay of digital marketing efforts and impacts search engine optimization, social media marketing, pay-per-click advertising, content marketing, and various other segements of digital marketing depending on what platforms are being relied upon.

Based on a similar term used in the financial world.