One cannot go past hundreds, maybe few thousands MAU without a significant hype and mainstream attention.
Mastodon was the decentralised social network and main ex-Twitter alternative at the time. Misskey attracted Japanese artists from a specific theme , exiled by Twitter. Pixelfed benefitted with Instagram's moderation drama and then with a Streisand effect. Its autor helped it with some marketing but this only helped with the migration wave, not caused it. Lemmy and /kbin grew on Reddit API debacle. PeerTube is one of self-hosting video solutions and a P2P video platform and a federated one. NodeBB and WordPress carried over their legacy user bases.
No other platform has more than several thousands MAU