Pretty much only whatsapp groups from what I've seen. I'd guess there are a disproportionate amount of millionaire on HN than other online communities of this size. I know at least 10 people who daily HN and have a net worth > $5MM.
As you would expect. I'm in one with a few dudes who are worth over a bil given their exit sizes, all folks who have IPO'd cloud companies. It's got a very imaginative name "cloud guys" - it's mostly posting in linkedin urls and asking if anyone can make an intro, couple of them natter on about their kids nannies sometimes, it's very uninteresting. Thing about these rich people groups is they're actually super boring. Everyone is already rich so it's not like we're swamping trading strategy or whatever, pretty much everything is already managed in these peoples lives they already have access to people they wanna chat with, so there isn't a whole heck of a lot of discuss.