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SAN FRANCISCO - Four senior Twitter executives are leaving the media company, which is also adding a new chief marketing officer and two board members, Chief Executive Officer Jack Dorsey confirmed, outlining the biggest leadership changes since he returned as CEO, calling for "bold rethinking".

Media head Katie Jacobs Stanton, product head Kevin Weil, and the head of the engineering division, Alex Roetter, will all leave the company, Dorsey tweeted late Sunday.

On Sunday night Jason Toff, who heads Twitter's video streaming service, Vine, tweeted that he was leaving Twitter to join Google to work on virtual reality.

Some of the executives are departing under pressure and others decided on their own to leave the company, a source told Reuters. Twitter has been struggling to muster growth to please Wall Street.

The company will announce the two new board members soon, the person said.

Dorsey took to Twitter to address reports of the exodus.

Was really hoping to talk to Twitter employees about this later this week, but want to set the record straight now: pic.twitter.com/PcpRyTzOlW — Jack (@jack) January 25, 2016

Weil, Stanton and Roetter are among the highest-level departures in recent months, but several executives have left Twitter since co-founder Dorsey stepped in as interim CEO in July. He has since been named Twitter CEO.

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In an earnings conference call in October, Dorsey spoke about "hiring and investing in talent" and the need for "bold rethinking."

Dorsey, who is also CEO of financial services technology company Square, has yet to lay out a detailed plan of what he wants Twitter to do differently. Dorsey plans a retreat with Twitter executives this week, the source said.

Weil has been at Twitter since 2009, although he has only served as its product head for about a year. He has often served as a face of the company, speaking at conferences and hinting at new developments on the site, such as a character count that would exceed the current 140-character limit.

Twitter has shuffled through three product heads since 2014, and Dorsey has played an increasingly large role in product development.

Stanton joined the company in 2010 and was tasked with growing its audience outside the United States. She previously worked at Google Inc and Yahoo Inc as well as the White House and State Department.

Twitter had its slowest user growth last year - it now boasts just over 300 million users - and was eclipsed by photo-sharing app Instagram, owned by Facebook Inc , which surpassed 400 million users last year.

Twitter's stock has fallen nearly 50 percent since Dorsey's return and is now trading below its IPO price. Analysts said there was concern Twitter had failed to grow its user base even with a quickened pace of product rollouts under Dorsey.

Since then, Dorsey has launched Moments, a product developed by Weil, which showcases Twitter's best tweets and content, laid off 300 workers and hired former Google executive Omid Kordestani as executive chairman.

The news of the departures was first reported by the technology news site Re/Code.