Image caption Evan Williams has been the chief executive of Twitter for the last two years

The co-founder of social networking site Twitter is standing down as its chief executive.

Evan Williams, who has headed the company since 2008, is being replaced to allow him to focus on ways of making Twitter more profitable.

Announcing the decision on Twitter's blog, Mr Williams said Twitter needed to "meet its potential as a profitable company".

He will be replaced by the current chief operating officer, Dick Costolo.

Twitter has grown dramatically in recent years, with user numbers now topping 160 million, compared with three million two years ago.

But Mr Williams said that in order to continue to be successful, the company now needed to start converting that growth into profit.

"Growing big is not success in itself," he said.

'Important transition'

"Success to us means meeting our potential as a profitable company that can retain its culture and user focus while having a positive impact on the world. This is no small task.

"This is why I have decided to ask our COO, Dick Costolo, to become Twitter's CEO. Starting today, I'll be completely focused on product strategy."

Mr Williams is the second of Twitter's founders to have served as chief executive, having taken over from Jack Dorsey in 2007 when he moved to become the company's chairman.

The BBC's technology correspondent in Silicon Valley, Maggie Shiels, said the handover marked a "very important transition" for Twitter.

"The announcement shows this is a company that is about to get really serious about making money... something they have tiptoed around for some time," she said.

"This is about turning Twitter from a communications platform into an advertising platform and a real money-making business.

"That means it will be able to fight off being acquired by the likes of Google and also challenge the search giant for lucrative ad dollars."