>Mastodon looks to have around 800k active users. For comparison IRC has (according to netsplit.de) around 280k users. Is that successful?

Yes.

Bear in mind many people here would consider geminispace to be a success and I seriously doubt that it even has 100k users.

"Success" has valid definitions beyond market capture and revenue. Mastodon is a success because it hosts a community and because it represents a validation of the model of decentralized federated social media.

And it isn't a zero-sum game, either. The entire point is that there doesn't have to be one "Twitter" one "Facebook" one "Youtube," or even one protocol to rule them all.