Twitter's Dick Costolo to step down amid stalled growth and troll troubles

• Co-founder Jack Dorsey to take over as interim chief executive
• Costolo had faced controversies including piracy and online abuse

The outspoken Twitter chief executive Dick Costolo will step down in July, the company's top brass said on Thursday, even as he insisted the sudden departure - which arrived after years of stalled user growth and amid a storm of controversy - was his own decision.

Related: Twitter CEO: We suck at dealing with trolls and abuse