I'm pretty big into the mechanical watch community. Insta used to be popular in that community, but the best content and discussion has long since moved off "social media" and into various private Slack and WhatsApp groups.
I've heard of a similar movement in other hobbyist/enthusiast communities.
For people I actually know IRL, it's virtually all WhatsApp.
Bill Watterson graphically murdered a dude in the funny pages and no one blinked an eye. Amazing.
Musicolet is such an awesome app.
I moved to Google Messages when Signal dropped SMS support and it's been fine. Most people I know who aren't on Signal or WhatsApp are on iPhones, but there are a decent number who apparently have RCS so -- perversely -- Signal dropping SMS support had actually resulted in a net increase in my secure messaging.
Agreed on spying, though I don't think it's any worse than posting on public sites.
The new people concern cuts both ways. Yes, it's moderately harder for brand new folks to learn about them and gain access. But it's far from impossible, and the additional hurdle or effort tends to substantially cut down on trolls, spammers, etc.