How did the compute power increase 15x ? Last I tried to join a server, they were full.

The number of active servers increased by 300 in the same timespan, so the influx was spread out a bit. The strain on the bigger servers has been a little rough, but it should level out.

Your numbers are a bit off. It's 7m total up from ~5.6m before all this, not active. The active count historically drops back down to ~100k after booms, but this one does seem different and sustained due to the different circumstances. It's usually a small burp in a specific community. It's never been all of Twitter's most active users from across all communities considering alternatives.

It's one thing to get users. It's another to keep them. Mastodon's user experience is godawful. It might be an IRC replacement for nerds, but it'll never be Twitter.

I've found Mastodon's UI to be so vastly superior to Twitter that it isn't even funny. Volunteers are running circles around Twitter's UI team.