Mastodon has already become viable. I switched when there was court decision that Musk had to buy twitter and deleted my account there. At that time Mastodon was ghosttown - posts related to US were very very few and refreshing the page after a minute gave handful of posts. It changed as it achieved critical mass. By that time I had gotten used to mastodon which is not very user friendly and is different from twitter. Kbin/lemmy are much similar to reddit in that regard.

Now it does the main thing twitter did for me - telling me the news - what is happening around and what others think about that. And it is much better than twitter which was mostly clickbait. It lacks in terms of depth on specific topics, but general topics are there - though you have to know how to curate those.