I was on many of these networks, but something I see fairly common across many of them, is they were acquired or shut down by other large social networks.
I suppose if you are a big social network, you have the clout and money to buy out the competition more easily. The irony of it is that many of the big social networks themselves appear to have a vacuum inside them, where the number of user accounts is not the same as the daily active users. It’s rumoured that Facebook may already have more dead user accounts, than for the living. Big social networks have a lot of infrastructure and staff to support, which gets paid by advertiser revenue, so I suppose they need to keep showing the numbers and posting the adverts, and don’t worry too much if the users are not actually actively using the platform.
The thing is with social networks, for users it is about the social part, so they want to see posts from friends and about things that interest them. The network effect holds many back from joining new networks. But it should also be remembered that when Google, Facebook, etc started up their networks, they were actually open networks, often using protocols like XMPP to connect with friends even outside that network.
I hope that we go back to more interoperable social networks, so that users can switch networks or servers, without losing their friends. Maybe that will also allow smaller networks to coexist next to larger social networks, and provide more variety and choice for users.
See https://www.mentalfloss.com/posts/defunct-social-media-platforms