Instagram succeeded for that same reason: profiles set to private, no real name policy, people can't look you up in a search bar. It was easier to group up in small communities. The less that's the case, the crappier the content. Instagram was ruined the moment Facebook accounts were attached to Instagram accounts — it's just dying slowly, the same way Facebook is dying slowly.
The next "global" platform is going to be Discord. It started as a place to host gaming communities. People were subscribed to a few gaming communities, so it already made sense for them to join more communities that are available on the platform. Right now, it's no longer the place exclusively for gaming. Every subreddit has one, every Patreon supporter is a member of some secret one. It'll outlast both Instagram and Facebook for one reason only: no personal info what so ever. People can't find out anything about you by clicking on your username: not your real name, not your contact info, not even a list of other communities you are a part of. You join a community by being invited to one.