Jack Dorsey began his tenure last year as Twitter’s chief executive with layoffs and a string of new personnel appointments. It turned out those moves were just a prelude.

The social networking company is in the midst of a major overhaul of its top ranks in the next few weeks, from its eight-member board to key executives in engineering and product, according to people familiar with Twitter’s plans, who spoke on the condition of anonymity because the details were private. Mr. Dorsey confirmed some of the changes late Sunday night in a tweet.

The shake-up is taking place as Mr. Dorsey, 39, attempts to reshape Twitter. The social networking service, used by presidential candidates such as Donald J. Trump and sports stars like the basketball player Stephen Curry, has failed to attract droves of new users. That has investors questioning how useful the service is for people and whether it will become a mainstream hit on the scale of Facebook or Instagram. Shares of Twitter have plunged nearly 55 percent in the last year.

Mr. Dorsey has been pointed in his critique of Twitter’s problems and what needs to be done to solve the issues. “A greater clarity of our purpose, our objectives and putting the people that use Twitter first drives the urgency we now feel,” he said last year. “You should expect Twitter to be as easy as looking out your window to see what’s happening.”