There are clearly different degrees to which information that is presented in a public space is expected to be disseminated to strangers. Simply being "made public" doesn't necessarily invite invasive spying on every detail of your public actions
Exactly, there is no expectation of privacy in a public place, so your friends should go to a private place if they want a private conversation (and similarly not have public social media profiles), not talking within earshot of others and expecting privacy, that people would close their ears while one talks. Restaurants, stores, and other such areas are explicitly not private places, I can't tell you the number of times I can hear embarrassing stories with no extra effort in my part, simply because people talk loud enough to hear.
That is exactly what I'm saying so I'm not sure we disagree. Don't put things on social media you don't want people to see, or at least make your profiles private, because people can and will look and you can't expect otherwise.
We certainly agree that social media and in-person conversations in public spaces have the same expectation of privacy, but that's not the point I'm getting at.
What I am asking is, don't you think it would be considered inappropriate to spy on such conversations even though it's technically and legally possible to do so?
Our judgments as to what is considered socially acceptable behaviour aren't strictly ruled by what is technically or legally possible, nor should they be.