Observation
Social calendars have only two states: empty or full.
When you have tons of free time, trying to get together with other people is nigh impossible. You just don't get together with people often enough to get invited to things.
But if you have plenty of social activities, then people invite you to more. It's exponential growth!
I'm still amazed at how this works. The scale and speed at which networks operate is quite difficult to wrap one's head around.