Who are the predominant users of LinkedIn then? Marketing mostly, or is it recruiters? Startup founders? How do you measure "activity" in this scenario - is it public postings or are we including the private messages that people send about jobs?

I see a lot of useless posts end up on my feed - self congratulations and a lot of what equates to a press release for some product - but those have to target a user base of some kind that isn't there to just do that or it ends up being bots talking to bots (effectively), which is arguably bad for the company (e.g. bad for their valuation).

Seems like the largest userbase are mid-career white collar workers, even if they don't make up the majority (or even a plurality) of the public posts/interactions happening.

Similar to a gacha mobile game you need some minimum amount of userbase to create the environment where "whales" will be encouraged to engage with their money - in this case that's advertising revenue through sponsored posts and premium accounts.