Facebook is a social networking service that allows users to connect with friends and family online. It was launched in 2004 by Mark Zuckerberg while he was a student at Harvard University. Users can create a personal profile, add other users as friends, and exchange messages, including automatic notifications when they update their profile. Additionally, Facebook allows users to join or create interest groups, event pages and business pages, share photos and videos, and play games. It's one of the most popular website and mobile apps in the world.