I'm not sure about all the nuances and how things work with photos tbh and things are/should probably be treated differently when there is no monetary gain being made, as in the case of Facebook and Instagram. I think thats another case really, but I'm also not an expert on the matter lol.
If you're writing something new, like a whole new post, then it's not plagiarism either. The way I read it originally in your post, was that you are copy and pasting the same article on both sites, which is plagerism and which, in my opinion, doesn't look great.
I'm not trying to be the police here or anything just giving my opinion. Take it as you wish