(Telecompaper) Twitter CEO Dick Costolo has announced his resignation, on the same day the company decided to end the 140 character limit on direct messages, Bloomberg resported. The resignation will take effect from 1 July. Co-founder Jack Dorsey will replace Costelo as interim, until a permanent replacement is found. Costolo came under fire from investors for failing to rejuvenate the service and for low customer uptake. He will leave without taking about USD 16 million in stock awards. The market welcomed the departure, with shares jumping in late trading. Dorsey's appointment signals the company will focus on improving its product, after a rake of leadership changes in the last five years. Direct Messages are different from tweets in that they are only visible between individual users. The change in the number of allowed characters will come next month.