By Sheila Dang (Reuters) -“Is Twitter dying?” billionaire Elon Musk mused in April, five days before offering to buy the social media platform. The reality, according to internal Twitter research seen by Reuters, goes far beyond the handful of examples of celebrities ghosting their own accounts. Twitter is struggling to keep its most active users – who are vital to the business – engaged, underscoring a challenge faced by the Tesla Inc chief executive as he approaches a deadline to close his $44 billion deal to buy the company. These “heavy tweeters” account for less than 10% of monthly overal…