[FkZ]

I think that's probably a large reason along with two others:

They're more content focused. Twitter is about what people write, Instagram is about the pictures people take. Facebook is about... the network? Facebook is more muddled, and clarity is valued today.

Asymmetry. Yes, I know you can "like" things and get updates and you can join groups, but ultimately the relationship between two users on Facebook has to be agreed to by both.