Dennis Crowley is a successful businessman known for his contributions to the technology and startup industry. He might be associated with founding or leading prominent companies.
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.