I know exactly what you mean and as you know I'm fairly new to the folk world, I came across similar 'folk' when involved with horses on the eventing circuit.
More recently friends of mine who act as servas hosts
introduced me to some of their guests - it was amazing, within an hour our kids were all playing together and we were chatting as if we had known each other for years.
So no I don't think it is common only to the folk world, but it's pretty cool when you find it!