The majority of my Facebook friends don’t use Instagram, and I’m not Facebook friends with the majority of the people I follow on Instagram. Heck, I don’t know many of the people I follow on Instagram. And like most normal people, I don’t accept Facebook friend requests from people I’m not actually friends with. (Unlike many people who work in tech.) Facebook is my Path, you might say.
Facebook is primarily used to share content with friends, exclusively. Instagram isn’t held to that standard.