Since then, the world's most popular social network has been redesigned dozens of times.
But its aim has remained the same: to connect people online. And make mountains of money from advertising.
As the platform turns 20, here are four ways Facebook has changed the world.
1. Facebook changed the social media game
Other social networks, such as MySpace, existed before Facebook - but Mark Zuckerberg's site instantly took off when it launched in 2004, proving just how rapidly an online site of this kind could take hold.
In less than a year it had one million users, and within four years it had overtaken MySpace - fuelled by innovations such as the ability to "tag" people in photos.
Taking a digital camera on a night out, then tagging your friends in dozens of pictures was a staple of teenage life in the late noughties. The constantly changing activity feed was also a big draw for early users.
By 2012, Facebook had surpassed one billion users a month and, aside from a brief blip at the end of 2021 - when daily active users dropped for the first time - the platform has continued to grow.
By expanding into less connected countries and offering free internet, the company has maintained and increased the number of Facebook users. At the end of 2023, Facebook reported it had 2.11 billion daily users.
2. Facebook made our personal data valuable… and less personal
Facebook proved that collecting our likes and dislikes is extremely lucrative.
These days, Facebook's parent company Meta is an advertising giant which, alongside the likes of Google, takes the lion's share of global advertising money.
On Thursday Meta reported more than $40bn (£32bn) in revenue for the last quarter of 2023, chiefly from offering advertisers highly targeted ad services. Some $14bn was declared as profit.
But Facebook has also shown where data collection can go awry.
Meta has been fined multiple times for mishandling personal data.
The most publicised case was the Cambridge Analytica scandal in 2014, which led to Facebook paying $725m to settle legal action due to a significant data breach.
In 2022, Facebook also paid a €265m (£228m) EU fine for allowing personal data to be extracted from the site.
And last year, the company was issued with a record €1.2bn fine by the Irish Data Protection Commission, for transferring European users' data outside of the jurisdiction. Facebook is currently appealing against the fine.
3. Facebook made the internet political
By offering targeted advertising, Facebook has become a major platform for election campaigning around the world.
For example, in the five months leading up to the US presidential election of 2020, incumbent President Donald Trump's team spent more than $40m on Facebook adverts, according to Statista research.
Facebook has had a hand in changing grassroots politics too - by enabling disparate groups of users to gather, campaign and plan action on a global scale.
Facebook and Twitter were said to be crucial during the Arab Spring in helping to co-ordinate protests and spread the news about what was happening on the ground.
But the adoption of Facebook for political ends has been criticised for some of its consequences, including its impact on human rights. In 2018, Facebook agreed with a UN report that said it had failed to prevent people using the platform to "incite offline violence" against Rohingya people in Myanmar.
4. Facebook kick-started Meta's dominance
With the huge success of Facebook, Mark Zuckerberg built a social network and technology empire that remains unprecedented in terms of users and its consequent power.
Up-and-coming companies, including WhatsApp, Instagram and Oculus, were all purchased and turbo-charged under the umbrella company Facebook, which changed its name to Meta in 2021.
Meta now says more than three billion people use at least one of its products every day.
And when it hasn't been able to buy its rivals, Meta has often been accused of copying them - in order to maintain its dominance.
Facebook and Instagram's disappearing Stories feature is similar to a key feature found on Snapchat; Instagram Reels is the company's answer to the challenge posed by video-sharing app TikTok; and Threads is Meta's attempt to imitate X, formerly known as Twitter.
Tactics have become more important than ever, thanks to the increased competition and a more strict regulatory environment.
In 2022, Meta was forced to sell GIF-maker Giphy at a loss after UK regulators blocked it from owning the service due to fears of over-dominance in the market.