Twitter Inc Chief Executive Officer Dick Costolo abruptly announced he was stepping down on Thursday amid increasing scrutiny of the company’s slow user growth and inability to attract advertisers at the same rate as its competitors. Costolo will be replaced by co-founder Jack Dorsey on an interim basis. According to a source familiar with the matter, it was Costolo’s decision to leave, and Costolo said he brought it up with the board last year as it began talking about succession planning.