Is Mastodon successful enough to be called "the future" of its niche? MAU is 1/3rd what it was at the peak, and bluesky + mastodon MAU combined is microscopic compared to twitter (I use none of these services, no dog in this fight, just looking at numbers).

Discord is not federated.

Twitter is full of bots, flamers, and scammers. I wouldn't trust their numbers.