Twitter has named Jack Dorsey, one of the company’s co-founders and the first person to run the Internet service, as its fourth chief executive.

Mr. Dorsey, who is 38, is also the chief executive and chairman of Square, a mobile payments company that is preparing for an initial public offering of stock later this year.

Despite initial concerns, big investors in both companies have grown comfortable with the idea of Mr. Dorsey juggling both jobs.

Mr. Dorsey has pushed a faster pace of product improvement while serving as interim chief since Dick Costolo, the company’s chief executive for about five years, stepped down on July 1.

On Wednesday, for example, the social media company extended the option of including a “buy” button in tweets to millions of merchants, including major retailers like Best Buy.

Next week, Mr. Dorsey and other executives are also expected to preview for advertisers a new video product and a feature that will show top tweets by subject, code-named Project Lightning.

Mr. Dorsey, who is also the chairman of Twitter’s board, faces the difficult task of figuring out how to attract new users to the company’s namesake social network while continuing to improve its advertising products. Twitter had an average of 316 million monthly users in the second quarter, a number that has been largely flat for some time.

Despite some changes to make the service easier for newcomers to understand, Twitter still lags other social networks like Facebook and Instagram and major messaging platforms like WhatsApp in popularity. It has also been slow to capitalize on the public’s enormous demand for video, although its new Periscope app has established a strong position in streaming live events.

Twitter’s challenges have unnerved investors, who were counting on it to match the rapid growth of its early days. Mr. Costolo, who became Twitter’s chief executive in 2010, stepped down after largely failing to meet Wall Street’s expectations after the company’s initial public offering of stock in November 2013.

Triumvirate at Twitter. Adam Bain, the company’s president of global revenue and partnerships, who was widely considered to be in the running for the chief executive job, will become president and chief operating officer. And Evan Williams, one of the company’s founders and Twitter’s chief executive before Mr. Costolo, will take over the chairmanship from Mr. Dorsey.

Twitter has become a household name, with political leaders and celebrities around the world using the network to broadcast text messages and images to millions of followers. When news happens, word often spreads first on Twitter.

But the company has struggled to convey to ordinary people why they should regularly use the service, which carries roughly half a billion messages a day. Twitter has long lacked a product visionary, although Mr. Dorsey has tried to play that role while serving as Twitter’s interim chief.

Twitter is caught in the classic chicken-and-egg dilemma: The service is largely useless unless you take time to choose relevant accounts to follow, but without obvious uses for it, no one will invest the time to set it up properly.

The company has tried to break out of the trap by offering casual visitors a taste of the “best of Twitter” on its home page before requiring that they create an account, and by setting up “instant timelines” of tweets based on who is on a person’s cellphone contact list. Similarly, it has begun exploring the possibility of selling ads that would be shown to more casual visitors.

So far, it is too early for those nascent efforts to show results.

Born in St. Louis, Mr. Dorsey has often said he grew up enamored with the niceties of densely urban areas — mass transit, maps and the nature of dispatch and communication. While he enrolled as a computer science student at the University of Missouri, Rolla, (now the Missouri University of Science and Technology) he left school and moved to New York in the mid-1990s, excited by the thought of living in Manhattan and working as a dispatcher at a bike messenger company.

After a brief stint at New York University, Mr. Dorsey left school just shy of receiving a diploma and moved to the San Francisco Bay Area in 2000. Soon after, he met Evan Williams, founder of Blogger, which Mr. Williams sold to Google, and of an Internet podcasting company called Odeo. With a handful of other people, the two would go on to create Twitter.

Twitter’s history over the last 10 years mirrors its current struggles; a revolving door of management and leadership, fighting for power among founders and, of course, Mr. Dorsey’s ouster from Twitter’s top job. He became executive chairman of Twitter’s board in 2008 when Mr. Williams took over the chief executive job.

Mr. Dorsey did not rest long after being pushed out of his own company. He went on to found Square, the commerce and payments company, with a small group of friends and entrepreneurs. Today, Square is valued at more than $6 billion.

But friends say Mr. Dorsey has been consumed with having a larger influence at Twitter, particularly as the company has struggled with slowing growth.

(NY Times)