Still, he intends to do it. Mr Zuckerberg claims that users will benefit from his plan to integrate its messaging apps into a single, encrypted network.
The content of messages will be safe from prying eyes of authoritarian snoops and criminals, as well as from Facebook itself.
It will make messaging more convenient, and make profitable new services possible. But caution is warranted for three reasons.
The first is that Facebook has long been accused of misleading the public on privacy and security,
so the potential benefits Mr Zuckerberg touts deserve to be treated sceptically.
He is also probably underselling the benefits that running integrated messaging networks brings to his firm,
even if they are encrypted so that Facebook cannot see the content.
The metadata alone, ie, who is talking to whom, when and for how long,
will still allow Facebook to target advertisements precisely, meaning its ad model will still function.
End-to-end encryption will also make Facebook's business cheaper to run.
Because it will be mathematically impossible to moderate encrypted communications,
the firm will have an excuse to take less responsibility for content running through its apps, limiting its moderation costs.
If it can make the changes, Facebook's dominance over messaging would probably increase.
The newfound user-benefits of a more integrated Facebook might make it harder for regulators to argue that Mr Zuckerberg's firm should be broken up.
Facebook's plans in India provide some insight into the new model.
It has built a payment system into WhatsApp, the country's most-used messaging app. The system is waiting for regulatory approval.
The market is huge. In the rest of the world, too, users are likely to be drawn in by the convenience of Facebook's new networks.
Mr Zuckerberg's latest strategy is ingenious but may contain twists.