One of our son's friends who we've known since they were in infants, is someone we are in very regular touch with. Growing up we often took him away with us to places like Center Parks, his father was never in his life. As an adult he has taken up golf and has been out on the golf course with my husband on several occasions, I think my husband has always been a bit of a dad figure in his life. He is probably the friend from the kids school days we are closest to, there have been others. This friend has grown up to be a very nice young man, the relationship changes in adulthood just as it does with one's own children.
I remember when I was growing up I spent a lot of time with at one particular friend's house, many of us did, her Irish parents were amazingly relaxed and kept an open house for their children's friends, it was very close to our town, and was a place where we congregated, they were incredibly tolerant about having lots of kids in the house. I was at her home as often as I was at my own, often stayed over. As we transitioned into adult hood, they asked us to use their Christian names. Mr and Mrs was the normal way of addressing friends' parents when I was growing up. At first that seemed quite impertinent. Lack of formality with children's friends was still a generation or so away, our children's friends called us by our first names right from babyhood.
We are not on FB, so contact with children's friends was pretty much on the basis of our children bringing them round to our house, or us bumping into them.
We've moved away now, but have told our son's golfing friend he will always be welcome in our new house once restrictions are lifted.