A social networking site is an on-line service that attracts a society of subscribers and provides such users with a multiplicity of tools for distributing personal data and creating subscribers-generated content directed to a given user's interest and personal life. Operators of online social networks are gradually giving out potentially sensitive information about users and their relationships with advertisers, application developers, and data-mining researchers. Some criminals too uses information gathered through membership profile in social networks to break people's Personal Identification Numbers, e-mail accounts and passwords. In this book, we analysed how private information can easily be made public in such sites. The book also looked at how it can be used to distort national security, how it became the new weapon of mass mobilisation and also how social networks have became the rallying forces for revolutions and social justice. At the end recommendations and countermeasures were made on how to safe guard subscribers' personal data.