I love a good tinfoil hat theory, but in this case I doubt it. I have FB blocked on my network via pihole, but I don't explicitly block Instagram. Until sometime late last week (I noticed on Saturday), blocking facebook.com also blocked Instagram. As of this weekend, Instagram works just fine even with those blocks in place.

I suspect Facebook was making some change to their DNS generally, and they made some kind of mistake in deployment that blew up this morning.