The evidence for this is https://www.caseywatts.com/blog/welcome-to-mastodon/ which is completely vacuous: a single person for an unspecified period measuring unspecified 'engagement' with a round number and no mention of how many followers cannibalized from Twitter or how a migration produces tons of fodder for discussion (eg the entire post is an advertisement/explainer for Mastodon) or about the general decline in Mastodon traffic post-exodus bump. Maybe establish that there is anything there before trying to explain as the will of the omnipotent Algorithm...