In December 2005, I joined Facebook.  At the time, it was still limited to college students.  Your status updates were only visible on your wall, and your friends had to come to your wall in order to post messages.  There were silly groups to join, and poking people and throwing sheep were popular interactions.  This was long before games, apps, pages, and marketing on Facebook.  In the time since, I have been very vigilant about only adding friends that I actually know.   Therefore, it is likely that many of my Facebook friends also know each other in real life, leading to a dense network.