If it’s FaceBook and it’s their main photo, people don’t always put their own photo up, until recently I had one of my Mum when she was younger.
My FB account is only open to my friends so they all knew it was her. But at the moment my photo is of my Mums family out on a day trip.
However maybe your memory isn’t so good as to what your friend used to look like, depending on when you last saw them? A couple of my old school friends look so different I would never have recognised them, my memory of them is from when they were much younger.