In a lot of cases it is deliberate and not something that "just happens". See enshitification or Instagram or what can happen in a marriage, say

https://www.amazon.com/Uncoupling-Turning-Points-Intimate-Re...

If you are starting a new social media service, for instance, the N^2 dynamics are brutal and you have to work so hard to attract, onboard, and retain each precious user. A site that has momentum is practically impossible to kill and, barring a really enlightened form of benign neglect (Craigslist?), you will eventually go into a "harvesting" mode for either money or social impact.