It’s not just social media content that is easier to predict after some time has passed. In 2018, Burcu Yucesoy and her colleagues at Northeastern University analysed the popularity of books on the New York Times bestseller list. Although it’s very hard to predict whether a given book will take off in the first place, books that do become popular tend to follow a consistent pattern afterwards. The team found that most books on the bestseller list saw rapid initial growth in sales, peaking within about ten weeks of publication, which then declined to a very low level. On average, only 5 per cent of sales occurred after the first year.[58]
Despite progress in understanding online outbreaks, most analysis still relies on having good historical data. In general, it’s difficult to predict the duration of a new trend ahead of time, because we don’t know the underlying rules that govern transmission. However, occasionally an online cascade does follow known rules. And it was one such cascade that first sparked my interest in contagion on social media.
Dressed in an ’i love haters’ baseball cap, the woman plucked the goldfish out of its bag and dropped it into a cup full of alcohol. Then she downed the drink, fish and all. A trainee lawyer, she was travelling around Australia and had performed the stunt after being nominated by a friend. The whole thing had been filmed. Before long, the video was posted on her Facebook page, along with an accompanying nomination for someone else.[59]
It was early 2014, and the woman was the latest participant in the online game of ‘neknomination’. The rules were simple: players filmed themselves downing a drink, posted it on social media, then nominated others to do the same within 24 hours. The game had swept through Australia, with drinks becoming more ambitious – and alcoholic – as the nominations spread. People downed booze while skateboarding, quad biking and skydiving. Drinks varied from neat spirits to cocktails that included blended insects and even battery acid.[60]
Coverage of neknomination spread alongside the game itself. The goldfish video was widely shared, with newspapers picking up ever-more-extreme stories. When the game reached the UK, it triggered a media panic. Why was everyone doing this? How bad would it get? Should the game be banned?[61]
When neknomination hit the UK, I agreed to examine the game for a BBC radio feature.[62] I’d noticed that during games like neknomination, participants transmitted the idea to a handful of specific people, who then passed it along to others. This created a clear chain of propagated transmission, much like a disease outbreak.
If we want to predict the shape of an outbreak, there are two things we really need to know: how many additional infections each case generates on average (i.e. the reproduction number), and the lag between one round of infection and the next (i.e. the generation time). During new disease outbreaks, we rarely know these values, so we have to try and estimate them. For neknomination, though, the information was laid out as part of the game. Each person nominated 2–3 others, and these people had to do the challenge – and make their nominations – within 24 hours. When I forecast the neknomination game in 2014, I didn’t have to estimate anything; I could plug the numbers straight into a simple disease model.[63]
My outbreak simulations suggested that the neknomination trend wouldn’t last long. After a week or two, herd immunity would kick in, causing the outbreak to peak and begin to decline. If anything, these simple forecasts were likely to overestimate transmission. Friends tend to cluster together in real life; if multiple people nominate the same person during the game, it will reduce the reproduction number and lead to a smaller outbreak. Interest in neknomination indeed faded quickly. Despite the UK media frenzy in early February 2014, it was all but gone by the end of the month. Subsequent social media games followed a similar structure, from ‘no makeup selfie’ photos to the widely publicised ‘ice bucket challenge’. Based on the rules of the games, my model predicted all of them would peak within a few weeks, just as they did in reality.[64]
Although nominated-based games have tended to fade away after a few weeks, social media outbreaks don’t always disappear after their initial peak in popularity. Looking at popular image-based memes on Facebook, Justin Cheng and his collaborators have found that almost 60 per cent recurred at some point. On average, there was just over a month between the first and second peaks in popularity. If there were only two peaks, the second cascade of sharing was generally briefer and smaller; if there were multiple peaks, they were often a similar size.[65]
What makes a meme become popular again? The team found that a big initial peak in interest made it less likely that the meme will appear again. ‘It is not the most popular cascades that recur the most,’ they noted, ‘but those that are only moderately popular’. This is because a small first cascade leaves more people who haven’t seen the meme yet. With a large initial outbreak, there aren’t enough susceptible people left to sustain transmission. For a cascade to recur, it also helps if there are several copies of the meme circulating. This is consistent with what we’ve already seen for stuttering outbreaks: having multiple sparks can make infections spread further.
Cheng looked at popular images, but what about other types of content? Back in 2016, I gave a public talk at London’s Royal Institution. Over the next couple of years, a video of the talk somehow racked up over a million views on YouTube. Around the same time in 2016, I’d given a talk on a similar topic at Google, which had also been posted on YouTube, on a channel with a similar number of subscribers. During the same period,
