I definitely agree that they will try, but I’m sure the average FB/Insta/Threads user won’t go through the process of installing a third party store just for those apps, particularly when they’re already available on the App Store. With something like Fortnite that’s not available I could see it. But with Meta products I doubt it, particularly with the older, less tech savvy crowd that tends to use them.
None are out yet, AFAIK AltStore just got approved but isn’t released yet.