This document discusses the development of social networking sites (SNS). It begins with early computer-mediated communication systems like USENET and bulletin board systems that allowed users to interact online. Popular early online services then provided chat features and communities. IRC, ICQ and instant messaging further enabled real-time communication. Early social networks included forums, SixDegrees, and LiveJournal. Major advances were sites like Friendster, Myspace, LinkedIn and Facebook that supported user profiles and connections between friends. Features like photo sharing on Flickr and video sharing on YouTube also grew popular. Twitter and Tumblr enabled real-time updates. Facebook expanded from colleges globally and added features like News Feed, video calling and apps over time.