I'm not on FB, I'm glad it's all a mystery to me.
I got together with my husband before social media existed. We were both 2nd time arounders. He had children and his grandchildren came along soon after we had our children together. So he had quite a lot of baggage from his marriage and I just had my cat from mine. We now also have grandchildren together. I was always in a very lucky position to get on with his children, sadly one of them died, but I regard my step-daughter, step-daughter in law, and step grandchildren as the very best of friends. When his parents were alive we went there often and they did have a picture of my husband, ex wife and themselves sitting around a table in evening wear at some function they all went to, quite honestly it didn't bother me, everyone has a past and I don't think it should be air brushed out. Somewhere in our house I have photos in an album of me and my ex, and of course, as they were a family my husband has a lot of his children growing up which include their mother. That's the way it is. All I would say, it seems to me that FB could drive anyone slowly mad in all sorts of way, I think the word used by you "fixated" is the key here, it's a bit like smoking, I'm glad I never took it up, but at it's worst FB seems pretty addictive in some ways with harmful effects. Try and wean yourself off it. I wish you happiness OP.