In February 2004 Mark Zuckerberg launched "thefacebook.com" from his Harvard dorm room, initially open only to Harvard students. Within three years it spread to U.S. universities and then to the world, passing one billion active users in 2012 and three billion in 2024. The other links of the same wave: Twitter (2006, Jack Dorsey), which moved onto pocket devices when the iPhone arrived in 2007; WhatsApp (2009), Instagram (2010), Snapchat (2011), TikTok (2016, ByteDance). Two shared decisions shaped the era: (1) usage was free, (2) revenue came from targeted advertising — meaning the product was the user's attention.

The 2010s showed the cumulative effect of those two decisions. A continuous information stream available on a phone reshaped human behaviour: by 2024 the average user was spending 2.5 hours a day on social media. "Algorithm" became an everyday word for the machine-learning rankers that decide which post is shown when. Eli Pariser's "filter bubble" thesis spread: when an algorithm pushes a user the views they already endorse, public debate fragments and polarises.

Political effects were visible quickly. In the 2011 Arab Spring the platforms acted not as organisers but as accelerators. In 2016 the Brexit referendum and the election of Donald Trump put the possible impact of micro-targeted campaign ads on the agenda. In 2018 the Cambridge Analytica scandal broke: data from 87 million Facebook users had been used for political profiling without consent. The same year UN reports documented Facebook's failure to police anti-Rohingya hate speech in Myanmar. In 2021 internal documents leaked by Frances Haugen — the "Facebook Files" — showed the company's own research linking Instagram to worsened body-image issues in teenage girls.

By the mid-2020s social media was no longer treated as neutral infrastructure; it was a core subject of regulation, litigation, and public deliberation. The EU's 2022 Digital Services Act, U.S. state bans on under-16 use (2024), and Elon Musk's 2022 acquisition of Twitter and rebrand to X all approached the same underlying question from different angles: can the world's public square be left to a handful of private companies optimising for ad revenue? There is no settled answer yet — but it is no longer in dispute that the design decisions taken between 2004 and 2012 shaped the political, cultural, and psychological climate of the 21st century.