On November 2nd, 2010, more than 61 million adults visited Facebook’s website, and every single one of them unwittingly took part in a massive experiment. It was a randomised controlled trial, of the sort used to conclusively test the worth of new medicines. But rather than drugs or vaccines, this trial looked at the effectiveness of political messages, and the influence of our friends, in swaying our actions. And unlike most medical trials, this one had a sample size in the millions.

It was the day of the US congressional elections. The vast majority of the users aged 18 and over (98 percent of them) saw a “social message” at the top of their News Feed, encouraging them to vote. It