I think people leaving facebook en masse would be a good thing (TM) for society, not a doomsday. Fewer echo chambers, less disinformation, and people forced to make effort to contact each other.
Hacker News is at least somewhat informative in the tech sphere. It tends to favor more esoteric technology like Lisp in lieu of things like Java & JavaScript which is far more favored on Reddit/Programming. Politics are mostly left leaning mixed with some Libertarian views. There is a good splattering of non tech stuff as well (music, art, literature, linguistics...etc). I'd say it's a far cry from fake news and cat videos.
This effect is partially muted by the fact that one can only see the vote status of one’s own post. There is still a tendency toward conformity, but at least the suggestion to vote with other readers isn’t apparent like it is on many content aggregators/discussion media.
I've been following Josip's work on and off for years now (he's probably on every big white hat hall of fame there is), and I'm pretty sure he wouldn't go public even if it took them a month to fix this.
If he said in a public blog post that it took them a month to fix something so simple, I could see the shit storm aimed at Facebook on social networks (including here), but I highly doubt any user would be compromised.