Where this one falls for Mastodon remains to be seen. It's more AOLish than Academic, in the sense that it's the result of a capricious event on a large outside-the-destination-system network (Twitter, in this case) rather than a regular seasonal influx (as with uni students discovering Usenet). It isn't however a wholesale dumping of an entire community onto another though as was the case for AOL in 1993.
Mastodon's adoption for at least the past six years has tended to be driven by failures and frustrations with other platforms, and Twitter in particular. Those seem to be occurring in increasing waves. There's some grumbling and adjustment with each, but so far, knock wood, Mastodon's assimilated the influx fairly well.
At the time of the September that Never Ended, Usenet was ~1m total users (based on data from John Quarterman's The Matrix (1990) citing DEC researcher Brian Reid, which I've referenced in earlier HN comments). I believe AOL at the time was at least of an equivalent magnitude, if not several times this.
But it's clear with the planned changes that they're turning paid blue check accounts into the equivalent of the ads you see at the top of your Google search, placed above 'organic' results.
What you might say is 'I don't think Mastodon is going to replace Twitter', but I don't think you can argue that Mastodon won't benefit from the Twitter exodus because it has already gained a million users.
Looking at the charts on https://the-federation.info/mastodon you can see that in May there was an influx of about 500 thousand new users after Musk's acquisition of twitter was announced. About 100 thousand of those stuck around. This time the number of new users is 1.1 million and still rising. So proportionally, the Twitter exodus will probably bring Mastodon at least 220 thousand new permanent users. My actual guess would be that a higher proportion will stick around this time, because of network effects.