Jack Dorsey has been serving as chief executive since founding the social media platform in 2006.
Elliott Management has been pushing for a sale of the gas station retail chain.
The appointment comes three months after Elliott Management hedge fund pressed for the ouster of Twitter CEO Jack Dorsey.
The agreement averts a battle over Twitter CEO Jack Dorsey's leadership but "lays the groundwork for eventually replacing" him.
Twitter shares jumped on reports that Elliott Management is seeking to replace Jack Dorsey because he also runs Square.
A record 99 campaigns globally had an M&A-related thesis, accounting for about 47% of all the year's activity.
Amid pressure from Elliott Management, the telecom and media giant announced a plan to cut costs and pay down debt.
“In the past few weeks it became clear that I was not on the same page as my new board,” CEO Devin Wenig tweets.
The activist investor believes AT&T is deeply undervalued, citing its "irreplaceable assets, enormous earnings power and an ability to win in key markets."
After years of decline, the bookseller was sold for $476 million.
The German software giant has prioritized revenue growth but is now taking steps to boost the profitability of its cloud business.
eBay's shares rose 6% after Elliott Management disclosed a 4% stake in the company and unveiled a plan to unlock strategic value.
