Twitter CEO Jack Dorsey will step down as head of the social media platform he co-founded a decade and a half ago, the company announced Monday. The surprising news comes after Dorsey returned to serve a second tenure as chief executive and guided the company through some of its most tumultuous years.

CNBC was first to report the news on Monday, with Dorsey later posting on the platform that it was "finally time for me to leave."

"I want you all to know that this was my decision and I own it," Dorsey said. "It was a tough one for me, of course. I love this service and company."

Dorsey navigated his company through an incredibly turbulent era. As CEO he decided to permanently boot then-President Donald Trump, who regularly utilized the platform as a soapbox. During his tenure, he faced increasing calls for regulation from lawmakers around the world.

The entrepreneur, 45, cofounded Twitter in 2006, and the company has helped make him a billionaire. Today his net worth sits at $12 billion.

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The former CEO still owns 2% of Twitter, worth roughly $1 billion, and continues to serve as the chief executive of Square. The bulk of Dorsey’s massive fortune comes from his 13% stake in the digital payments company. Dorsey was the only person to be CEO of two public companies with market valuations of greater than $5 billion.

Parag Agrawal, former chief technology officer, was appointed CEO effective immediately. Dorsey will remain at the company as a member of the board until his term expires next year, the company said.

"I've decided to leave Twitter because I believe the company is ready to move on from its founders," Dorsey said in a statement released by Twitter. "My trust in Parag as Twitter's CEO is deep. His work over the past 10 years has been transformational. I'm deeply grateful for his skill, heart, and soul. It's his time to lead."

Dorsey had been facing investor pressure from critics who have long complained that his dual CEO arrangement has divided his attention, usually to Twitter's detriment. Investment firm Elliott Management went as far as to launch a campaign to remove Dorsey as CEO last year.

Dorsey will continue to operate as the chief executive of Square and advance his philanthropic endeavors, where he has already pledged to donate $1 billion.

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Sources: CNBC