Recent years have seen exponential growth of social network sites. They flatten the real-world social network by making personal information and social structure visible to users outside the ego-centric networks. They provide a new basis of trust and credibility upon the Internet and Web infrastructure. For the vast majority of social networks, it takes only a few clicks to befriend other members. People's dynamic ever-changing real-world connections are translated to static links which, once formed, are permanent. The existence of static links as public exhibition of private connections causes the problem of friendship inflation. There is mounting evidence to support the idea that there has been an inflated number of digital friendship connections on most SNSs. The theory of friendship inflation is also evidenced by our nearly 3-year observation on Facebook users. Friendship inflation can devalue the social graph and eventually lead to the decline of a social network site. There have been rise and fall of many social networks, which we argue is caused by friendship inflation. The book proposes a novel algorithm, called ActiveLink, to identify meaningful online social connections.