Instagram users will no longer be able to message Facebook users starting this December.
Meta in 2020 said that Facebook Messenger will be merging with direct messaging on Instagram, which allows Instagram users to initiate chats with Facebook users and also the other way around. The plan was to merge WhatsApp, Instagram, and Messenger into a single messaging platform with a goal of keeping people within the ecosystem and staving off competitors like iMessage and the others.

Recently, however, it seems that the plan has been abandoned as a new support page says that cross-app communication between the two platforms will no longer be possible. Once this functionality ends, those on Instagram will have to initiate a new chat via Facebook if they wish to have a conversation.
Meta has not commented on why they’re ending cross-platform chat, but it could be due to its interest in entering the EU market and complying with the region’s Digital Markets Act.