In my experience, what happened is that young people eventually stopped posting personal updates (this was a multi-year process), either because the novelty of it declined or because they entered a life phase change (e.g. leaving college) that made social updates less frequent or less shareable.
In my friend group, this happened around 2018, about a year after we left college and entered the working world. I'd guess something similar happened with many cohorts.
For most people I know, they stopped posting when the feed started being filled with suggested crap. We didn't want to post on a feed we hated and where our friends would likely miss it anyway amongst all the garbage.
LinkedIn (or is it LInkedIn?) still serves one purpose: you can sign up for a free month of lInkedLn premium and drop a priority one chat message in someone's inbox. Much better than getting lost in someone's spam folder.
In my friend group, this happened around 2018, about a year after we left college and entered the working world. I'd guess something similar happened with many cohorts.