Condensed from Wikipedia -
Facebook is an online social networking service. Users must register before using the site, after which they may create a personal profile, add other users as friends, exchange messages, and receive automatic notifications when they update their profile. Additionally, users may join common-interest user groups, organized by workplace, school or college, or other characteristics, and categorize their friends into lists such as "People From Work" or "Close Friends." In September 2012, Facebook had over one billion active users.
Facebook has regularly caused controversy since its inception over privacy concerns and changes in practice. Currenty concerns include declining organic reach and a push for businesses to pay to have posts reach followers. Other concerns include fears that paid advertising does not reach real users, but 'click farms' and artificially generated response.