There's demography but also the fact that if I have a site with 1,000 users I can dream of a total addressable market of 1,000,000,000 or so. The most important thing is to get enough users to "prime the pump" where they start filling out content and there is somebody to interact with.
Circa 2000 or so the usual advice to give the business people was "base your register and login flow on Yahoo" because Yahoo had done the usability work and people were familiar with it. By 2007 or so this was terrible advice because Yahoo was facing terrible problems with account takeovers, smurph accounts and controlling bad behavior and was making login harder, not easier.
I think Facebook is in the same place today.
I nuked my Facebook account in 2016 but made one a year ago to go with my Meta Quest 3. It's amazing how bad the "new user" experience is when they don't have any data on you -- on the other hand, they don't care about onboarding anymore because the stragglers like you are people who are creating a Facebook account for a reason. Instagram, on the other hand, should be proud of their cold set because they really do have a few 100 really amazing things to show you on day one. If only they realized that (i) I don't want to engage with OnlyFans models, and (ii) if I did it would be more like Ely EEE than the ones they show me...