I am going to say, don't give up on potentially learning who some of the "unknowns" are, I have found several of mine. I had one, of a family and was at one of the libraries going through their family files and found a couple of the couple (man and wife) in the photo made the same day. Same outfits, hair styles. I was able to add another photo into the family file and also concretely identify my photo. So, it does happen.