Co-founder Biz Stone leaving Twitter

Stone, 37, said Tuesday on his blog that he will work with the company "for many years to come," but that the most effective use of his time now is to "get out of the way" of Twitter's crew and leadership team until he's called upon to be of some specific use.

Stone says he plans to focus on helping schools, nonprofits and company advisory boards. He's also relaunching Obvious Corp. with fellow Twitter co-founder Evan Williams to develop new projects.

The move comes as Twitter has been trying to build upon its popularity to make more money by selling more ads. The privately held company doesn't disclose its finances, but research firm eMarketer Inc. estimates Twitter will bring in advertising revenue of about $150 million this year.

Stone's departure caps a year of executive changes at the San Francisco-based company. Last fall Williams handed over the reins to Twitter's current CEO, Dick Costolo, and moved on to explore new business ideas. In March, Twitter's third co-founder and original CEO Jack Dorsey returned to oversee product development as executive chairman.

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