7 deadly sins marketing

Why is social media popular?  Why do people spend all that time uploading/tagging photos, developing online presences in the form of flickr albums, facebook pages, second life avatars etc.?

“Some technologies and applications are age-specific, but narcissism is not one of them,” said Clay Shirky, a new-media professor at New York University and the author of a forthcoming book on social media called 'Here Comes Everybody' - “The pleasures of the self reaches all demographics.”

A widget developer (oh - ok - slide.com) jokingly suggested that they can predict an app's success through the lens of the seven deadly sins - that is, if the potential app didn't effectively leverage one (or more) of them, it wouldn't catch on. 

According to Wikipedia, the seven deadly sins are as follows: Luxuria (extravagance, later lust), Gula (gluttony), Avaritia (greed), Acedia (sloth), Ira (wrath), Invidia (envy), and Superbia (pride). 

Which one do you think makes the most effective marketing tool?