Costolo's decision to step down is a shock on one hand and no surprise on the other. Twitter's internal politics have always been a bit brutal according to some authors but this shows that no-one is safe.
Costolo got Twitter through its share issue but couldn't then get the growth and profits those shareholders wanted.
Some leaders seem better suited to different lifecycle stages in the development of a company. Maybe it's start-up, maybe is listing the company, maybe it is winding it down. There is no inherent reason why one leader should be able to lead through all these various stages.
That is a challenge to owners, board members, non-execs and investors. They need to challenge the person in charge and make sure they are the right person for the stage of development that the organisation is in.
Dick Costolo is stepping down as chief executive of Twitter, the company has announced. The social messaging service's co-founder Jack Dorsey will take over as interim chief on 1 July and stay until a replacement can be found.