On 23 April 2013 US news agency Associated Press tweeted that the White House had been attacked and then-president Barack Obama injured. The tweet was a hacker-produced fake but it led to the Dow Jones falling 130 points and a loss of $136 billion from the S&P 500 index before it was deleted.
"That one tweet changed the game," says Anthony Cheung of trading educator Amplify Trading. Before then, he explains, "[no traders were] really using Twitter - it was seen as trashy, unauthenticated news". But, after seeing how the White House tweet moved the markets, many more traders began to pay close attention to the microblogging site.
They're wise to do so, a number of academic studies produced in the years since then have suggested. One of the most recent of these - a research paper published last year by Professor Ting Li, Dr. Jan van Dalen and Pieter Jan van Rees of Rotterdam School of Management - found that big data analysis of tweets can be used to predict developments in the stock market in both the short and long term.
After collecting more than 1.2 million tweets about S&P 100 companies over seven months, the academics discovered that companies that were the subject of bullish tweets experienced higher than average returns. They also noticed that the number of tweets about a particular stock are related to short-term trading volumes, volatility and returns.
Most importantly for traders, they concluded that analysis of Twitter could "be used to exploit market inefficiencies profitably, even when transaction costs are taken into account".
Their findings are not isolated ones. A 2015 New York University study found that tweets about a company before earnings announcements could predict both earnings levels and market reactions to them. In 2016, the US's National Bureau for Economic Research reported that a trader only relying on Twitter would be almost four times more profitable than one using a standard trading approach.
In 2016, the US's National Bureau for Economic Research reported that a trader only relying on Twitter would be almost four times more profitable than one using a standard trading approach.
Cheung agrees that Twitter is invaluable for traders today. "Nearly every professional proprietary trader uses Twitter these days," he says. "It's essential for what we do."
Banks and hedge funds often choose to buy access to one of a number of systems monitoring Twitter that have sprung up in the last few years. These include big data specialist Ravenpack, Twitter-owned Gnip, and Dataminr, which is one of the few that still has access to the full "firehose" of tweets since the Gnip acquisition.
However, intelligent use of Twitter is also within the reach of individual traders. Cheung advises traders to follow news sources on Twitter closely, especially key journalists. Doing so can mean finding out about big political or economic events minutes ahead of those monitoring traditional news outlets, which can be easily enough to turn a trading profit from market reactions.
Cheung also suggests using free Twitter tools such as Tweetdeck to stay on top of key hashtags or geographies, which can yield useful insights for trading.
Finally, Cheung recommends keeping a close eye on very influential people who tweet frequently. This advice is backed up by the Rotterdam academics, who note that the correlations they saw between tweeting activity and share price movements were magnified when they gave more bandwidth to particularly authoritative tweeters.
However, as the now-notorious spoof White House tweet shows, traders using Twitter to shape their trading strategy need to be able to sift accurate information from fake news. Like other social media sites, Twitter is regulated much less than conventional publishers. The SEC monitors social media activity as part of its efforts to combat market fraud - but it would be impossible for any regulator to catch every guilty party.
However, thinks Cheung, Twitter is self-regulating to an extent. Any tweeter discovered to be putting out misleading information, especially if they did so to further their own financials interests, would immediately lose all credibility with serious traders.
Another reason traders should stick with Twitter is perhaps its most famous user: a certain @realDonaldTrump. Whatever you think about his politics, the extensive use of the platform by the US president has made his account essential reading for traders, says Cheung - the world's most powerful leader has the potential to influence markets significantly.
It's unlikely that any White House tweets in the future will be quite as shocking as 2013's fake Obama attack tweet. But given that President Trump has nearly 50 million followers (the Associated Press has just 105,000) he may well spur a few market surprises - and therefore trading opportunities.