The vast majority of commenters here fundamentally misunderstand what Twitter is, or rather what it has become.
It's not a "social network" anymore. I mean, it is in the technical sense - you and I can both sign up for an account. But what it is today is an elite coordination mechanism, and Twitter became this because it was the first of it's kind and some sort of momentum or technological inertia put us where we're at. Journalists, politicians, business people, elites more generally etc. all use this thing to coordinate what they think about basically everything.
So you're really asking "how do I create another semi-public elite coordination system that could step in if Twitter falls apart".
It's not a "social network" anymore. I mean, it is in the technical sense - you and I can both sign up for an account. But what it is today is an elite coordination mechanism, and Twitter became this because it was the first of it's kind and some sort of momentum or technological inertia put us where we're at. Journalists, politicians, business people, elites more generally etc. all use this thing to coordinate what they think about basically everything.
So you're really asking "how do I create another semi-public elite coordination system that could step in if Twitter falls apart".