While I'm not sure I agree with the GP's premise...

...Gen Z doesn't have to be "using" anything particular. Just because, for some years, "everybody had to be on Facebook" doesn't mean that the decline of that means something will necessarily replace it.

There was no "previous Facebook", and there doesn't have to be a "next Facebook".

There were a half a dozen or more "previous Facebook." MySpace, Friendster, LiveJournal, DeadJournal, etc. There were so many during the MySpace era that I've forgotten most and barely recall the bigger ones. There will be a replacement but hopefully it will look different.

Hah, should've known I could count on HN to be super pedantic about offhand statements like that...

MySpace was founded in 2003, Facebook in 2004. Yes, there was a brief period where MySpace was more "the place to be" than Facebook, but that was essentially a flash in the pan, given that Facebook has held that crown now for so many people for over a decade.

To expand a bit on what I said in the GP post:

In 2002-4, the early social networks all started to come online. Facebook was the only one of those to survive with any relevance. It has now existed for 18 years, and its relevance among young people is definitely on the wane.

There was nothing like it before that time: Usenet, forums, AOL chatrooms, and BBSes didn't give anywhere near the same kind of connection or experience that Facebook circa 2008 did.

There is no inherent reason that there must be something like it afterward. TikTok isn't really comparable to Facebook; it doesn't lend itself to the same kind of communication or connection. Twitter (which may now be dying) and its decentralized imitator Mastodon may come closest, but they don't encourage the kind of long-form posts that Facebook does, nor do they have the same ability to build out your IRL social network in the digital space that was the hallmark of early Facebook.

There may, at some point, be some new company, service, or protocol that can to some meaningful extent replicate the experience of Facebook in its heyday; or that can, at least, command the attention and interest of as many people...but there's no guarantee that that will be the case, and everyone looking for X, Y, or Z to be "the next Facebook" are begging the question by assuming that there will be.

I'm not being pedantic, I'm disagreeing with you. Friendster (2002) was wildly popular. LiveJournal (1999) was huge. The things you list (Usenet, BBSes, etc) we're the precursors but they were not social media where LiveJournal was (to an extent) and Friendster is arguably the original. There was something before Facebook and MySpace and there will be something after. Facebook is neither unique enough nor important enough to hold in this level of esteem. It is and always has been derivative.

IgorPartola mentioned MySpace. When Facebook came out, I wondered why anyone would make a MySpace clone. I think there's an argument that there was a previous Facebook.

Absolutely yes regarding MySpace. Per Wikipedia: "in June 2006, it surpassed Yahoo! and Google to become the most visited website in the United States" https://en.wikipedia.org/wiki/Myspace

They use Discord and WhatsApp. “Social networks” as such turned into basically games of meme-meme-ad and that is an awful experience. Social is lost there entirely, it’s just a machine for getting your attention with memes and then showing you ads instead. If you want to communicate with people in any meaningful way, FB is useless. So Gen Z has been using tools that actually help them stay in touch with people and form relationships rather than staring at ads all day. I did a pool recently of a live audience of about 40 18-22 year olds. Two have Facebook accounts, one to pull up her baby pics her mom posted on there and the other doesn’t use his at all. The rest didn’t bother registering. Instagram I think was 4 people, who only use it as a messenger. That’s it.