Icahn, now in his late 70s, is one of the longest running activist shareholders and had been among the ranks of so-called corporate raiders in the 1980s, waging contentious battles against companies like U.S. Steel and Texaco. His arsenal now includes tweeting his opinions, and he tweeted that he was pleased with the buyback and earnings.
Apple's earnings per share also show the effects of the buyback. According to S&P/Capital IQ analyst Howard Silverblatt, Apple set a quarterly stock buyback record of $18 billion, and its earnings per share growth was 15.2 percent while its income grew less than half of that, at 7.1 percent. Apple reduced its quarter share count by 3.4 percent.
Some activists leave quickly and others stay for the duration. Icahn, appearing on CNBC this week, said he might consider committing to holding onto a stock, like Apple, for a year, and that he would announce it on Twitter.
Some activists become active players in companies whose shares they own.
Joe Sullivan is the CEO of Legg Mason and works beside activist investor Nelson Peltz who is on his company's board and supported his nomination last year. "I can only describe my relationship with Nelson. I have no interaction with other activists. The key word is constructive," he said. "My approach as CEO with Nelson has been to be transparent and straight forward and to leverage his experience and his thoughts and his team's experience and thoughts and capabilities as we think through things."
"He's demonstrated that he's not short-term oriented, that he thinks about the long-term value for shareholders," Sullivan said.
"My experience with Nelson and with Trian is they don't become dogmatically entrenched on anything other than doing what's best and making money for shareholders. We really have what I would call a constructive relationship with Trian," he said.
—By CNBC's Patti Domm.