...Facebook would shift away from public sharing of posts and focus on private conversations across the company’s messaging services...
Facebook will be little more than a chat/text function which is available to most people through their cellular phones for free, mostly, as part of their service with their provider.
A small niche market for those without that service or without a cellular phone.
A Facebook status post is already fully customziable in terms of who can see it. What else would they do with it?