Between the three, Facebook is literally everyone I've ever shaken hands with at a conference or kissed on the cheek at Easter. Twitter seems to be everyone I am entertained by or I wish to meet some day. Foursquare seems to be everyone I run into on a regular basis. All three of those social graphs are powerful in their own.
To the casual observer, the Dropbox demo video looked like a normal product demonstration, but we put in about a dozen Easter eggs that were tailored for the Digg audience. References to Tay Zonday and 'Chocolate Rain' and allusions to 'Office Space' and 'XKCD.' It was a tongue-in-cheek nod to that crowd, and it kicked off a chain reaction.