I was a highschool senior when Facebook came out, back when you needed an email with a university's domain name to sign up. That one was driven by pure FOMO. Everyone's older friends were on it and going to cool frat parties and you were just stuck on the outside looking in. LiveJournal and Myspace had nothing on that.
Call it a tipping point, call it a collective action problem finally being resolved, snowballing, viral liftoff, or belittle it and call it a "herd stampede" if you really want.
But those are all "organic" phenomena. The grandparent comment that called it "inorganic" seems to be mistaken.
There is this cynical (and maybe often warranted) take that any sudden uptake or virality is astro-turfing or I inorganic or shilling.
Not the case here though, I see it myself how basically all my old Twitter communities have migrated and things are really vibrant. Block lists, mute words and starter packs mean I we exactly what I want to see, which adds to the appeal.