Is it so much the individual maturing, and not just the platform itself losing relevance? The whole nature of these social platforms seems to be entirely ephemeral - they spread by word of mouth because others are using it. The problem comes in when you consider that a person isn't going to use more than 2 or 3 of these platforms. So as soon as a new platform is released, older platforms are guaranteed to lose users even if the new platform is worse, as it will take a number of weeks or months for a typical user to understand and then reject the new platform. During that time, the older platform isn't being used.
This exact issue seems to apply to online gaming as well, so this isn't some new discovery of human behavior. There are some 'big' games (warcraft, cod) that retain their users even after a decade - but they do this by reinventing themselves and recreating their products constantly. I think social media and gaming are similar enough for a lot of these behavior traits to apply to social media also - which means the correct approach is to redesign/improve core parts of your social platform at least once a year to keep it relevant.