It was the tweet that added $Apple's share price and will no doubt encourage a new generation of activist investors.
In fewer than 140 characters, veteran corporate crusader Carl Icahn sent the iPhone maker's shares to a six-month high and made himself $50m in a day.
"We currently have a large position in APPLE. We believe the company to be extremely undervalued. Spoke to Tim Cook today. More to come," he said via Twitter on Tuesday.
Icahn is one of the most prominent US activist investors – fund managers who buy stakes in companies and put pressure on management to increase returns to shareholders or shake up operations. So far, his discussions with Cook, Apple's chief executive, seem cordial. A follow-up tweet said they had "a nice conversation". But Icahn is prepared to get personal, the same approach he has taken in his battle to thwart Michael Dell's attempt to buy back the computer maker he founded.
Icahn wants Apple to borrow money cheaply to buy back shares despite the company already announcing a $100bn shareholder payout – comprised of buybacks and dividends – after pressure from another activist investor, David Einhorn.
Icahn has a spring in his step. Figures from Activist Insight, an information service, show that the average return on investments bought by Icahn since 2010 is 41.6%, compared with a 19.2% gain by the S&P 500 index over the same period. Those returns have been powered by Icahn's investment in Netflix, which has jumped 176% in value since he took a stake in October 2012.
Icahn's success is probably all the sweeter when compared with the fortunes of rival activist investor Bill Ackman. The pair have had a hostile relationship in the wake of a disputed deal a decade ago. Since 2010, bets by Ackman's Pershing Capital Management fund have performed better than Icahn's. But Icahn's recent performance is stronger after Ackman made two particularly bad calls.
This week, he left the board of JC Penney, the clothes retailer, after making losses of $700m on his investment and hiring then firing former Apple retail boss Ron Johnson as chief executive. Johnson's failed revamp caused a 25% fall in sales and a $1bn loss last year.
More embarrassing still is Ackman's decision in December to "short" Herbalife, a US nutrition and weight-loss company, by buying and selling $1bn of shares he did not own in the belief they would fall. The bet pitted him against Icahn and George Soros, who bought shares in Herbalife instead. Those shares have gained 77% since the end of March and Icahn has gloated that Ackman faces the "mother of all short squeezes", because he faces the prospect of buying back Herbalife stock at a vastly inflated price.
In a TV bust-up in January, Icahn called Ackman a "cry baby" and a "major loser" who tried to manipulate the market by shorting Herbalife and talking down the shares. Ackman responded that Icahn was a "bully" and "not an honest guy". This may be entertaining but debate is raging on Wall Street about whether activist swoops give a welcome jolt to complacent boards or are opportunist raids that force harassed companies into decisions that are bad in the long run.
Martin Lipton, the doyen of US corporate lawyers, argues that activists disrupt companies and threaten to weaken the economy by forcing boards to choose risky actions and investor payouts over long-term investment. "No company is too big to become the target of an activist, and even companies with sterling corporate governance practices and positive share price performance, including outperformance of peers, may be targeted," he wrote in a recent blog.
Activist Insight said the number of activist raids in the US was not rising at present but they were happening to bigger companies. Cheap debt markets and large corporate cash piles provide an obvious rationale for calling on boards to pay out more to investors. "Activists are now more likely to seek dividends or share repurchases compared to targeting poor corporate governance or business strategy," Activist Insight's managing director Kerry Pogue said.
The UK has a less entrenched tradition of using rapid stake-building to pressurise boards. The most high profile assault on a UK company in recent times was Knight Vinke's campaign against HSBC in 2007. Eric Knight's fund used a small stake in the bank to call for boardroom and pay reform and the axing of unprofitable businesses in HSBC's sprawling empire.
HSBC attempted to shrug off Knight's campaign but his stance struck a chord with fund managers who found HSBC high-handed. The bank brought in fresh directors and a new management, which has shut or sold peripheral businesses.
Julian Franks, a professor at London Business School, said activism generally increased the share price of the target company. "The activist on average does what the market would like them to do. The question is not whether the activist is too short term but whether the market is too short term. Whether markets put enough weight on long-term investment is an important question but I'm not sure I blame the activist." The impact can last longer than 140 characters.
Key players
Carl Icahn made his estimated $20bn (£13bn) wealth in old-style corporate raiding, but the 77-year-old is embracing new technology.
His tweet announcing his Apple stake promises to be one of many. Icahn has also focused on other tech companies.
In 2011 he turned his attention to Motorola, which then sold itself to Google for a 63% premium. On top of his stake in Dell, he has made more than $1bn from his stake in Netflix.
Raised by teacher parents in Queens, New York City, Icahn graduated from Princeton with a BA in philosophy and started work as a stockbroker four years later. He founded his own firm in 1968 and began taking stakes in individual companies 10 years later.
His hostile takeover of airline TWA in 1985 cemented his reputation as an aggressive operator as he sold the carrier's assets to pay off the debt he had used to buy it.
Bill Ackman has made losses of $1bn on JC Penney and Herbalife, but do not write him off yet.
His 21-year investment record is littered with big gains and crunching losses. His $1.8bn loss for investors on a stake in retailer Target in 2007 was "one of the greatest disappointments", he said. But in 2010 he made $1.5bn by helping rescue US mall owner General Growth Properties, while last year's boardroom coup at Canadian Pacific railways preceded a near-tripling of the share price.
The 47-year-old launched his first investment firm, Gotham Partners, after finishing his Harvard MBA in 1992. A failed joint bid for the Rockefeller Center attracted investors and Gotham took off. But he wound the fund down in 2002 before launching Pershing Square two years later. Many big bets and a $1.2bn fortune later, Ackman is unbowed.
Apple is expected to host its next 
