This is a very interesting article (hat tip to Michel) on why Facebook (and for that matter other social media platforms too) want you to have more friends. In essence it is because more friends equals more activity which equals more content. Keeping the content coming is the key to a living social network. Like a shark, that must keep moving forwards to stay alive, social networks that start to run dry of content, start to die:
Online social networks are built on user-generated content. Without this content, these networks are the equivalent of dying blogs (or MySpace). That said, Facebook faces the (potentially impossible) task of keeping its users engaged and active. Account holders have lives outside of Facebook, what social scientists call opportunity costs, so these social networks need to incentivize participation short of paying people. What better way than to give us a large captive audience of acquaintances, colleagues, classmates, friends and family to share our content with.