Twitter is expected to have approximately 400 million global users by 2018, mainly in Asia, Latin America, and the Middle East, according to an estimate made by consulting firm eMarketer.

The predicted number is far short of the once expected 1 billion users, suggesting that Twitter's user growth will have plateaued in major developed markets the next four years.

User growth in Twitter's key locations, like the United States and Japan, will have declined to around 6.4 and 6.1 percent, respectively. Its global user count will have reached just 386.9 million users, the digital advertising industry research firm said.

The U.S. accounts for three-quarters of the social media's revenue, according to Reuters.

The San Francisco-based company was once called the "next Facebook," as it was believed to be capable of easily reaching over 1 billion users. Instead, Twitter has struggled to increase its users during the first two quarters as a public company.

User growth is expected to accelerate in Asia however, according to eMarketer, who believes the social media company has "significant potential" in emerging markets.

India and Indonesia will likely pass the U.K. with the third- and fourth-largest Twitter populations this year, according to the firm.

Twitter could see a 60 percent user growth in India by the end of 2014.

One unknown factor that could affect forecasts is a change in Twitter's status in China, where the site, along with Facebook, is banned, according eMarketer.

Twitter Chief Executive Dick Costolo visited China for the first time ever earlier this year, and met with local government officials. Despite the trip, the company has downplayed the idea that it would be launching operations there sometime in the near future.

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