I cannot endorse this overbroad definition of "privacy" and in fact I can't see the logic in it at all. It is true that in the past the cops could not automatically hoover up your posts, but is also true that in the past no individual person could reach millions/billions of other individuals instantly with short messages and videos. The increase in scope of police collection of public communications is proportional to the increase in reach of those communications platforms. It is all well-balanced.