Twitter co-founder Evan Williams has asked Dick Costolo to become CEO of the company. This way Williams can focus more on the product. Currently Twitter has 300 employees. There was about 20 employees two years ago when Williams took the CEO job at Twitter.
Back then about 1.25 million tweets were created per day and now 90 million tweets are being sent out per day. Two years ago when Williams joined as CEO, there was about 3 million users and now there are 160 million users.
In the past, Costolo founded Feedburner, an RSS feed analytics company that Google acquired for $100 million. He quit Google in July 2009.