15 years ago, on February 4, 2004, 19-year old Harvard student Mark Zuckerberg, along with some of his classmates, launched a little website called Thefacebook. 

"Thefacebook is an online directory that connects people through social networks at colleges", the original website greeted its visitors back then, explaining that it would enable them to "search for people at your school, find out who are in your classes [sic], look up your friends’ friends and see a visualization of your social network." 

Confined to the Harvard campus at first, the service quickly gained popularity and was subsequently rolled out to other colleges. By the end of 2004, Thefacebook had more than 1 million registered users and the newly-founded company behind it had not only moved to Silicon Valley but also secured a $500,000 investment from PayPal co-founder Peter Thiel. After changing its name to just "Facebook" in 2005, the social network was opened to the general public in the fall of 2006, marking the start of one of the most remarkable growth stories in corporate history. 

Within three years, the social network added 350 million users and in the summer of 2012 it became the first online service of its kind to reach 1 billion monthly active users. By the end of 2018, that number had grown to 2.32 billion people, meaning that more than half the world’s online population is using the platform dreamed up by a teenager in his college dorm room.