Since the links in social networks are power-law distributed some people have mind numbingly large numbers of friends while most of us have many many fewer. Your much more likely to know these highly friendly people compared regular folks. Should you count the number of friends of your friends and compute an average you’ll almost certain to discover that average is greater than the number of friends you have. So generally speaking your friends are just plan friendly than you are.
That presumably leads to everybody feeling as if they don’t have as many friends as they ought to. Everybody’s impression is that they are a wall flower.
This insight was gleaned from M. E. J. Newman’s paper “Ego-centered networks and the ripple effect – or – Why all your friends are weird” found here.