Would you post the work of another author, albeit work that is available as Free Commons, without attributing it to that author?
Recently, someone I know did this on Facebook with 'Dust if You Must' by Rose Milligan. In fact, the poem stood alone in the post without any comment from the person who posted it. Unsurprisingly, the post was praised by several people, who asked if the poster had written it. In response, another person gave the poem the correct attribution, and suggested that the original poster should have done so. Instead, they replied that they felt that since it was their personal, 'private' Facebook account, they did not need to cite the author. They did not add the author's name to their original post, although when another person asked if they had written it, they did say 'no'.
No social media account is that personal or 'private'. Once something is posted, it can usually be shared and shared again, even if you only post it to friends and family. Perhaps in the instance above, this is what the poster intended, but it isn't what they did.
As a writer themselves, I would have expected them to have too much respect for the efforts and talents of another not to acknowledge it, especially as the poem clearly resonated with them. I don't believe any self-respecting author would ever intentionally mislead people into believing someone else's work was their own, but that was obviously what happened, and they weren't in a hurry to put the record straight.
This was plagiarism by omission, if not intent. If a writer is generous enough to allow anyone to use their work via Free Commons, it doesn't mean it should be anonymous. At the very least, correct attribution would allow the reader to research what else that author/poet has written. Writers should know better!