Why is Twitter's CEO stepping down?

Twitter chief executive Dick Costolo will step down on July 1. The social media company's first chief executive, Jack Dorsey, will take over on an interim basis.

Twitter chief executive Dick Costolo will step down on July 1, driven by pressure from investors frustrated by slow growth and recent controversies surrounding the social networking company.

Mr. Costolo will continue to serve on company’s Board of Directors, Twitter said in a statement. Co-founder Jack Dorsey will take over as interim CEO as the board conducts the search for a new chief.

The shakeup comes as Twitter struggles to attract new users at the rate of other social-media sites.

Twitter’s user base is 255 million worldwide, but its growth continues to slow down. eMarketer estimates that Twitter’s monthly user base will grow by 14.1 percent this year; just two years ago, growth topped 30 percent.

Meanwhile, the Twitter platform has been involved in recent controversies, including the illegal video streaming of the pay-per-view prize fight between Floyd Mayweather Jr. and Manny Pacquiao via the company’s new video platform, Periscope, and problems with trolling and bullying among its user base.

Twitter admits that it has failed in addressing the bullying issue. "It's no secret and the rest of the world talks about it every day," Mr. Costolo wrote in a February memo to his staff obtained by The Verge, a technology blog. "We lose core user after core user by not addressing simple trolling issues that they face every day.”

Costolo repeatedly said that the departure was his call in a meeting with company shareholders Thursday. 

“One thing I do want to make clear is that this transition is not the result of anything more than Dick deciding to move on from his role as CEO,” Mr. Dorsey told the investors.

Costolo joined Twitter in 2009 as the company's Chief Operating Officer. He was named the CEO of the company in 2010.

In company’s Thursday statement Costolo said he is “tremendously proud of” the Twitter team and all that they have accomplished during his six years in the company. As part of his departure, he has agreed to cancel all of his remaining unvested equity in Twitter after July 1, Reuters reported.

Costolo also endorsed Dorsey as the upcoming CEO. “There is no one better than Jack Dorsey to lead Twitter during this transition," he said during the shareholder meeting. "He has a profound understanding of the product and Twitter’s mission in the world as well as a great relationship with Twitter’s leadership team.”

Dorsey is Twitter's co-founder. He became company’s CEO in 2007, but stepped down in 2008 and founded Square, a mobile payments company.

Costolo’s Thursday tweet about the transition was brief: