Twitter's Chief Executive Officer Dick Costolo is stepping down, as the company tries to define its vision. Seperately the company is removing the space limit in its private messages. Twitter Inc. said Chief Executive Officer Dick Costolo is stepping down, reflecting the social-media company’s struggle to define its vision in the months following its initial public offering.

Jack Dorsey, a co-founder, will take Costolo's place in the interim until the company can find a replacement, Twitter said.

"Jack has a profound understanding of the product and Twitter’s mission, as well as a great relationship with Twitter’s leadership team, which will facilitate a smooth transition," the company said in a statement. Costolo will remain on the San Francisco-based company’s board.

Beyond the leadership shake up, Twitter also made a small announcement today related to the direct messages on the social network.

Form now on, Twitter is removing the space limit in its private messages. Direct Messages are different from tweets in that they are only visible between individual users.

"We’ve done a lot to improve Direct Messages over the past year and have much more exciting work on the horizon," Sachin Agarwal, Twitter’s product manager for Direct Message. "You may be wondering what this means for the public side of Twitter. Nothing! Tweets will continue to be the 140 characters they are today."