If I had to guess, you or your ex let it slip to someone in your personal circle. If you had FB, you likely had a friends list. FBencourages users to report these things. It would had been a complaint from someone with a link to your SO page where they could compare pictures.
I would not be able to trust most of my relatives to keep it quiet. They are a bunch of goody two shoes. Nobody uses FB without adding people they know. Isn’t that the point of it?
I got kicked off match.com in 24 hours because someone recognized my pic.
But all that aside, at least you won’t face charges if you use FB if this ruling goes our way.