Twitter co-founder Evan Williams has stepped down from his position as CEO.

In an announcement on the Twitter blog, Williams revealed that he asked Twitter COO Dick Costolo to step in as the company's CEO. Williams will stay with the company though and "be completely focused on product strategy."

From his announcement:

"The challenges of growing an organization so quickly are numerous. Growing big is not success, in itself. 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. I frequently reflect on the type of focus that is required from everyone at Twitter to get us there.

This led to a realization as we launched the new Twitter. I am most satisfied while pushing product direction. Building things is my passion, and I’ve never been more excited or optimistic about what we have to build.

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."