That to cut down on Twitter's site outages, slowdowns and bugs, while providing a sturdier and more flexible platform to develop new features and scale capacity more quickly.

Dorsey also on his Twitter account, saying he was "thrilled" with his return to the company as product development chief.

Williams, who will remain on Twitter's board, is developing an idea for a new startup company, and for that reason has been spending less time at Twitter lately, .

Now that it may have put its notoriously spotty uptime and availability problems behind it, Twitter faces the challenge of generating advertising revenue that is proportional to its skyrocketing popularity. Twitter launched its Promoted Tweets advertising program about a year ago.