For Costolo, the changes are designed to surround him with handpicked deputies to help stem two straight quarters of slowing user growth, said a person familiar with the matter. Twitter's board, which includes co-founders Jack Dorsey and Evan Williams, has raised questions about member growth and user engagement. Costolo has been concerned about building a stronger culture at Twitter and has been asking others for advice, one person familiar with the matter has said. Power Moves In April, Costolo hired Google maps executive Daniel Graf as vice president of consumer products, followed by the May appointment of a new engineering chief, Alexander Roetter, who was already at the company. Last month, COO Ali Rowghani resigned after a power struggle with Costolo over responsibilities, people with knowledge of the matter have said, and the role is going unfilled.