Evan Williams, the co-founder and chief executive of Twitter, is stepping down to lead product strategy at the company, Twitter announced on Monday. Dick Costolo, the chief operating officer, will succeed Mr. Williams.

The company, which announced the change in a blog post, is navigating a period of rapid growth and beginning to build an advertising business.

In the two years that Mr. Williams has been chief executive, Twitter has grown to 300 employees from 20; the service’s registered users have ballooned to 160 million from three million; and the company has started to generate revenue.