Evan Williams says Twitter's growth 'just getting started'

Even though Web start-up Twitter has gone from 1.6 million to 32.1 million users in a year, CEO Evan Williams says it's "just getting started."

"We think we can grow a long term sustainable valuable business here, and we're just getting started." Williams said in a CNBC interview today. "We're very mission focused."

If he and co-founders Biz Stone and Jack Dempsey wanted to sell the company, they would have already done so, he said.