Twitter is overhauling its top management as it struggles to boost profitability and growth prospects. Four senior executives left the company yesterday, CEO Jack Dorsey said in a tweet, adding that they had "chosen to leave". He said he was "sad" to see them go.But sources told the Wall Street Journal that at least two replacements would join the board this week.The newspaper reported that, as a condition of returning as boss last year, Dorsey had said all board members had to be replaced.Dorsey is credited with coming up with the idea for Twitter and ran it in 2007-2008 but was pushed out by shareholders because he was reportedly a better software engineer than a boss, and was known to leave work early for personal pursuits, such as sewing or yoga classes.Twitter stock recently hit a record low and the number of users has been increasing more slowly than the company would like. At the end of September, it had 320million active users - only 4million more than three months earlier.Twitter has been the subject of long-running takeover rumours - with Apple, Facebook, Google and News Corp named as potential buyers.Diagnosing causes for Twitter's malaise and prescribing its cure have become a parlour game for the tech industry. Twitter has failed to expand its user base beyond a core of politics, sports and celebrity junkies.But there was a time before its stock market flotation in 2013 when Twitter executives would talk of competing with Facebook for global scale. But while Mark Zuckerberg's juggernaut has powered ahead beyond 1.5billion active users, Twitter has stalled.There are signs that Twitter 's user base is losing interest. ..