Announcing the news, understandably in a Tweet, he said Twitter had to “meet its potential as a profitable company”.  He will be replaced by current Chief Operating Officer Dick Costolo.

Twitter, which was founded only four years ago now has 160 million users compared to only three million in 2008.  Mr Williams said though that “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.  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.”

Mr Williams took over as CEO from Twitter’s other co-founder, Jack Dorsey, in 2007 when Dorsey became the company’s chairman.

This is a sign that Twitter knows that recently they’ve just been treading water in social networking and that they need to raise their game in order to maintain their status and popularity.  It will inevitably mean a move to a more advertising-oriented business model, though how Twitter’s users will take to this remains to be seen.  There are also questions over the how advertising would best work on Twitter when a great many users only access the service through third-party software or a smart phone.

Will this mean that in a couple of years we’re all bombarded by Twitter Spam.  While this is unlikely, it’s clear that the future of Twitter will be very different from the service we all use today.

Someone made an effort to put all of the published Ted Talks in a list with direct onsite links to the videos, the name of the talker, the title, a description, the date, running time and event it was held. This is an incredible useful spreadsheet for anyone interested in at least one of the focus areas of TED.

Below are three embedded Ted Talk videos. The first is by Evan Williams, a founder of Twitter who is talking about how Twitter’s growth was driven by unexpected uses, the second showing Dan Ariely talking about why people are thinking that stealing and cheating is ok (sometimes) and the last by Tim Berners-Lee who is talking about the next web of open, linked data.

The list of TED talks contains 500 entries so far and it seems to be updated regularly with new content. This one is definitely worth keeping an eye on.