Williams is Twitter's second CEO – after fellow co-founder Jack Dorsey – to resign from the job after a two-year term.
Twitter's executives have yet to find a way of making money from the phenomenally popular microblogging service, but have recently started experimenting with methods including advertising via the Promoted Tweets service.
Meanwhile, VoIP firm Skype has announced the immediate resignation of chief Josh Silverman, who has been in the job since March 2008.
It has named Tony Bates, the head of Cisco's enterprise, commercial and small business division, as CEO, effective at the end of the month. CFO Adrian Dillon will serve as interim chief until Bates joins the company.
Bates’ role heading up Cisco’s enterprise group has been taken by CTO Padmasree Warrior.
The shake-up comes at a crucial time for Skype - its owner, eBay, plans to float its subsidiary on the Nasdaq this year via an IPO that could be worth up to $100 million.
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