These guys don't think like that. They think about the future - not the past. By any means, David Marcus was killing it at Messenger.
Rather than going to Amazon - you could just tell a chat bot to buy you a book. I don’t think the vision has come true at all.
(I don't think that Facebook would actually want to be the retailer in this transaction - that would involve competing with Amazon at something that Amazon is very good at and Facebook knows nothing about, like warehousing, shipping, purchasing, etc... and requires billions of dollars to build. So I don't see Facebook having any role except being a middleman.)
I can understand asking Alexa to restock the cereal if I buy the same brand all the time, but buying anything unique like a book, or an item of clothing or electronics, would seem to require much more interaction than such an interface is capable of.