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I wonder would you have not downvoted the post if he had mentioned the source or given credit to the original article?

I'd still downvote.

you can share content by teasing it, usually articles start with a short "teaser" that is also used for other social media channels. you can copy this and add a link to the full article

By teaser, I guess you mean that one should write something about the article in their own words to create some interest to click and go to the given link.