Facebook. Founded in 2004, Facebook is currently the biggest social networking service based on global reach and total active users. Launched by Harvard student Mark Zuckerberg and some of his contemporaries, the service was initially only available to Harvard students but soon expanded to regional universities, the Ivy League and further universities before opening up to high school students and global users aged 13 or over. Currently, the Facebook usage penetration among internet users in leading markets is above 80 percent.
Facebook users must register before using the social network and are free to create a personal profile in order to interact with other users which they can add as friends. Furthermore, Facebook users may join user groups based on workplace, college or school and can also categorize their Facebook contacts into lists. Users can post status updates or other content and message each other. Facebook users can also interact with a wide selection of applications including social games or other services like the photo-sharing app Instagram.
In September 2012, Facebook announced over one billion active users across all platforms. The social network is also accessible to mobile web users and has published a series of apps based on original Facebook features such as Facebook Messenger, which as of the third quarter of 2015 has a 37 usage penetration rate.