You probably see them every day, yet you don’t know them. They are called familiar strangers.
They are the people you meet repeatedly on the bus, on the tube, at the gym or at the cafeteria, but you never talk to them. Nevertheless they are a part of your social circle, because you kind of notice if they’re missing. You establish a relationship of sorts with them, one that dictates no contact, but no hostility. And if you happen to stumble upon them outside the normal routine they are associated with, maybe on holiday, then the social curtain lifts, and you do actually talk to them. Your common background becomes a strong enough connection to engage in social contact.
You probably have a few of them as Facebook friends, too. Would you say ‘hello’ if you encountered them in the street?
Berkeley University launched a project about this a few years back. It stems from the research of great cognitive psychologist Stanley Milgram, famous for his Small World Experiment that spawned the ‘six degrees of separation’ theory. He has explored this in a paper called The Familiar Stranger: An Aspect of Urban Anonymity.