The micro-blogging social media company Twitter has announced that its chief executive is stepping down. Dick Costolo had been under pressure from investors over the company's slow growth.

A statement released by Twitter on Thursday said that Costolo, who has been the Internet company's CEO for the past six years, would be replaced by co-founder Jack Dorsey on an interim basis.

Dorsey, who is to succeed Costolo on July 1, is only expected to hold the post as interim CEO until a more permanent replacement has been found.

"I am tremendously proud of the Twitter team and all that the team has accomplished together during my six years with the Company ... There is no one better than Jack Dorsey to lead Twitter during this transition," Costolo said.

"Welcome back, @jack," Costolo, who is to continue to serve on the board of directors, said via his Twitter handle.

A link to the official announcement from Twitter did not provide a reason for the change at the top of the San Francisco-based company. However, management has been under pressure due to Twitter's slower growth in terms of users, compared with other social media platforms such as Facebook and Instagram.

Twitter under pressure

Twitter owned 1.6 percent of the $50.7-billion (45.1-billion euro) US digital advertising market in 2014 compared with 1 percent in 2013, according to data firm eMarketer. That pales in comparison to Facebook, which increased its share from 7.6 percent in 2013 to 10.4 percent the following year.

The market research group projects that Twitter's monthly user base will grow at 14.1 percent this year, compared to more than 30 percent two years ago. Twitter's stock has also lagged some of its peers since its closing price after its first day of trading. Since Nov. 7, 2013, it has dropped 20 percent, compared to a 72 percent increase for Facebook, 9 percent increase for Google and 27 percent increase for Yahoo.

Chris Sacca, one of Twitter's earliest investors, posted an 8,500-word manifesto earlier this month calling on the company to better engage its users. He simultaneously praised the company's strong acquisitions of Periscope and TellApart, but Sacca pointed out that one billion users had tried Twitter and left the service. In a recent CNBC interview, Sacca said that Twitter would be an "instant fit" for Google if it were to acquire the microblogging service.

In a tweet Thursday, Sacca wrote, "In under five years as CEO, @dickc grew Twitter from a $3 billion valuation to a $23 billion valuation. Credit where credit is due."

Twitter reaffirmed its outlook for the second quarter of 2015, expecting revenue of $470 million to $485 million and adjusted earnings before interest, taxes, depreciation and amortization (EBITDA) of $97 million to $102 million.

The company disappointed investors in its first quarter results when it reported that its number of monthly average users was growing at a slower pace than Wall Street's expectations.

uhe/ng (Reuters, dpa, AFP)