Twitter co-founder Jack Dorsey has been named the new Twitter CEO -- a job he was sacked from just 7 years ago. His predecessor, Dick Costolo, quit the social network just months after he admitted that Twitter "sucks" at dealing with abusive behaviour and content. This marks Dorsey's second stint as chief executive since he helped start the San Francisco company more than nine years ago with Evan Williams, Biz Stone and Noah Glass.