By Gerry Shih SAN FRANCISCO (Reuters) – Twitter Incs messaging service will have close to 400 million global users by 2018, mostly in Asia, Latin American and the Middle East, consulting firm eMarketer estimated on Tuesday, far short of the 1 billion once expected. The new study by the digital advertising industry research firm suggests that Twitters user growth will have plateaued in major developed markets within five years. By 2018, user growth in Twitters key markets of the United States and Japan will have steadily declined to 6.4 and 6.1 percent, respectively, while its global user count will have reached just 386.9 million users, eMarketer said. The United States accounts for three-quarters of Twitters revenue, the messaging service disclosed last year.