Former Microsoft executive Steve Ballmer tweeted on Thursday night from an unverified account praising Twitter's "innovation" and mentioning he had recently taken stake in the company. The 4 percent purchase, which he confirmed to Bloomberg, makes him one of the company's biggest shareholders, now investing more in the company CEO Jack Dorsey, who reportedly owns 3.2 percent of Twitter's shares.
As word of Ballmer's investment got out, Twitter's stock skyrocketed. Fortune's Mathew Ingram noted at that at one point it had gained about $700 million—nearly as much as the $800 million investment itself—and it's only grown from there, with Twitter's market cap currently up more than $900 million over yesterday. Overall, Ballmer's vote of confidence in Twitter and its new CEO, cofounder Jack Dorsey, fueled the jump, but it's not the first time a single tweet has created a big shift in the stock market.
Just last month, Hillary Clinton tweeted she would soon lay out a plan to address skyrocketing prescription drug costs in response to the scandal involving Turin Pharmaceuticals CEO Martin Shkreli. Biotech stocks fell soon after.
In 2013, the Associated Press's Twitter account was hacked and tweeted that two explosions had hit the White House, sending the Dow Jones Industrial Average plunging 150 points in minutes. At the time, one trader told Bloomberg it was "quite scary" markets reacted to the news so quickly.
Tweets haven't always worked in Twitter's favor: There was the time last year that Twitter CFO Anthony Noto accidentally tweeted what was meant to be a DM about acquiring a company, and the time a poor earnings report leaked (where else?) on Twitter. Perhaps the company could take a page out of Apple's book, whose CEO Tim Cook once slowed a stock market slide with a single email. In any case, it all stands as a reminder that the stock market is a fickle beast.
Correction: This story originally said Ballmer's Twitter investment was worth $800 due to an editing mistake. It's actually around $800 million. $800 for a 4 percent stake in Twitter would be one hell of a deal.