Zuck appears to be experiencing another privacy epiphany. In "A Privacy-Focused Vision for Social Networking" he describes how he plans to revamp chat for ubiquitous e2ee and ephemerality of data, focusing more on chat as the primary mode of interaction on Facebook.

In the part about encryption he also described what is usually called "lawful intercept" and so what this probably means is an implementation of the ghost listener scenario described by GCHQ in which an extra hidden participant is silently added to the chat, unknown to the users.

He also describes the location of data being important, but only in terms of decisions about where to build data centers.

Although it's a long post all it really describes is something similar to the state of XMPP today, but without any onion routing possibility and non-federated with Zuck still very much in control of the data.