Gregory's Server
I think it made a lot of sense given their IG user base.
But this is a hype novelty moment. Whether the platform gets the basics right enough to be a Twitter substitute without the problems people have with it probably remains to be seen … incl how much IG culture can be transplanted onto microblogging without being crap.
Otherwise, yea, this could be a great fracturing of big social, which probably suits ActivityPub and fediverse well.