Twitter Inc’s announcement that Dick Costolo would exit as CEO on July 1 was long on plaudits but offered few clues on how the company will tackle its biggest problem: user growth. Twitter’ shares were down 0.6 percent in early afternoon trading on Friday, after rising much as 3.9 percent just after the opening, as investors assessed the company’s prospects. Analysts said Twitter has an uncertain future unless it can reverse slowing user growth by introducing new features that will help it win a bigger chunk of the digital advertising market from rivals such as Facebook Inc.