Fair use is never an entire news article. If the poster took one or two sentences that would be fair use. But that’s not what this is.

Fair use is more complicated than that. I see no hard line rule from the courts saying that a whole article couldn't be shared under fair use.

https://fairuse.stanford.edu/overview/fair-use/four-factors/

But also, it's a Facebook post. The author's not selling subscriptions to this content. He's not charging per download. He put it in the public square himself, and in the comments he even positively comments about it being shared more widely. I would be very surprised if the author felt as strongly about this as you do.

Not really, it's technically a copyright violation just like SciHub is.

Whether the rightsholder would consider this a problem or not is another question.