"But Facebook’s core social network is primarily a place to post photos and videos and comments that aren’t private and don’t disappear. That’s not the future Facebook is building toward"
https://www.msn.com/en-ca/finance/topstories/this-could-be-the-beginning-of-the-end-for-facebook-e2-80-99s-social-network/ar-BBUvOIO
FB / C-A / DARPA don't want citizens to have a memory-store / history that disappears
[Think of the recent embarassment the Pizza-style images brought to some Media personalities]
The pictures and records need to be ephemeral; erased routinely.
("1984", Citizens don't know what is being done to them because their history is blocked, and they mustn't remember nor communicate the contradictions of the fakenews and of Big Brother.)
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They don't want citizens to be able to broadcast to one-another. It's safer to have everyone broken up into little boxes / rooms.
That's their meaning of "private"