Icahn's goal is to get Apple to increase its $60-billion stock buyback program, announced this year.
Though the buyback is already the largest in history, Icahn wants to see it go as high as $150 billion, according to some reports. He argues that Apple can well afford a bigger buyback that would in turn boost the company's stock even further.
The latest tweet didn't indicate whether Apple's management was inclined to agree to his request. But his $1-billion investment apparently at least bought him dinner with the CEO.
Icahn's tweet came during a three-hour shutdown of stock trading on Nasdaq. Investors noted the aggressive investor's news but had to wait until nearly the end of the trading day to act on it.
Before the outage, Apple's stock price was down $3.83 to $498.53. But after trading resumed Apple finished the day at $502.96, up 60 cents.