Re: No
These were Facebook posts which were accessible to any Facebook user.
That category contains, at last count, somewhere in at least the mid-nine-figures -- let's say three hundred million, which gives us a reasonable number while still lowballing Facebook's own (possibly inflated, possibly not) estimates by a factor of around two and a half. That should give us a reasonable middle ground, I think. It's also, not by coincidence, also a reasonable approximation of the current population of the United States.
I think it'd be hard to argue that anything, accessible to three hundred million people, cannot be called 'public'. Of course not all three hundred million will be looking at it at once, and that's not in fact necessary to call it 'public'; the city of Baltimore, to pick an example, contains many fine public parks, each of which remains 'public' whether at a given moment it contains three hundred people, or a half-dozen, or none.
Of course, a physical analogy implies a problem of physical access, which does not exist in this case; as was established in the article, and as no one here has even tried to argue isn't the case, the Facebook posts in question were available, simultaneously, to about three hundred million people. If the whole thing had "gone viral" and been posted on the front page of everywhere, then every single one of those people could in theory be looking at it all at once. (Of course Facebook would go down if they all tried, but that doesn't impair the argument, because the posts would still be there for the rest of three hundred million to see as soon as Facebook came back up.)
In what way can you possibly argue that this place, metaphorical though it be, is not public? In your own example, it's "standing at a bus stop shouting through a megaphone big enough to reach a whole country's worth of people".
I mean, it's cute that you seem to want to argue all these bizarre hypotheticals, none of which remotely approach relevance to the case in hand. (Unlawfully compromising the content of a private conversation renders the conversation public? Are you really going to argue something that silly in front of God and everybody? I wouldn't -- hell, in my state, you can't even lawfully record a conversation in which you are participating without the explicit consent of all parties involved!)
But inventing your own facts, which you find preferable to those actually shown to exist in the context of the discussion, really isn't going to favorably impress anyone who is at all worth impressing.
All that said, you're right to note that we're still feeling our way into how this sort of thing works on the Internet. That we are doing so, seems no sufficient reason to abandon the principle, that we may expect to be held responsible for the provable effects of things we say or do. (Mostly 'do', even now; people don't yet routinely go to jail in the US merely for uttering racial slurs, to pick a recently controversial example. Give it a couple of decades, though...)