Right, I'm by no means under the impression that anything on the internet is private. I was, however, under the impression that people cannot take a photo you posted to an audience you chose and publish it on their public page.
To break it down (I'm going to try to give a similar example, to attempt to not spill dirty laundry on the internet), a couple of weeks ago, I took a trip and visited a museum. It was my and my friend's first time to the city, we were taking pictures of everything (I scrapbook all of my trips to notable places), and we took a picture on the back of the property, with the museum in the background. This person (who, may I add, does not follow me, nor have I ever met her - she got the screenshot from another person, since blocked), posted the picture and called me an f---ing racist, threatened to post my work profile and contact information, incited people to "condemn" me for visiting such a place and taking a photo, etc., etc. I really don't think I need to defend my actions, but my family are people of color, literally everyone at the museum was taking pics, etc., but this woman I don't know still felt the need to "call me out." Her post has since been retweeted numerous times, and I'm concerned for my future career and reputation. When several of my friends asked her to take the post down, her reply was that since I'm wearing sunglasses, my "identity" is protected. BS, since she exposed my workplace and a photo of me. But I digress.
I'm not sure if this constitutes defamation, but it was one option I was exploring. I'm not a confrontational person, so I don't want to sue her or try to make money off of this if I don't have to - I think the threat of a suit, if it was potentially valid, would be more than enough to get her to back down and delete the post. She's a bully, and doesn't have the funds for legal fees, so my guess is that would be enough. But I don't want to threaten something I can't back up, if needed.