I was thinking about this subject over supper, particularly as DS and family sometimes think about a job move elsewhere.
It occurred to me that one of the biggest problems with friends rather than family is that dense friendship/acquaintance support circles tend to be very localised. With a very narrow extended family DS & DDiL have a very complex social network made up of the children's non-family godparents, most of whom are unattached and regularly visit their god children and play an active part in their lives, other friends are those DDil has had since childhood as she still lives in the town she was born in friends through work, nursery, school, church etc etc. Were they to move this network would be fragmented and much of it would fall away and would have to be recreated from scratch in the new area they moved to. It would take the rest of DGC's childhood to build up a friendship circle comparable with the one they now have.
Family, however, remain with you and are interested in you and in touch no matter how mobile one's life is. I know we all have individual friends who have lasted years and much moving round, but it is the complex friendship circles we build up that for parents with young children in particular is so important.