I think the discussion about social media still exaggerates the popularity of Twitter. For sure it had the loudest discussions but it never reached the level of Facebook, Instagram, or TikTok, or even more closed instant messaging platforms in numbers (activity, revenue or number of users). Heck, in Finland, Twitter loses to many of the old style discussion boards and to comments sections of the big newspapers.
I feel that question “what replaces Twitter” is misplaced. Twitter never proved it could be long term sustainable. For most people they have the place already to meet people and for the Twitter diaspora it is a choice between going to popular platforms or embracing places like Mastodon or micro.blog.
I know what I will choose, I am happy with the audience of about one (myself) for my content and I am happy that I will never go viral.