Twitter users are expected to grow 24.4 percent this year to over 227 million with double-digit growth forecast to continue through 2018 when the number of users will be nearly 400 million, according to a report. The US will continue to be the country with the largest number of individual Twitter users through 2018, eMarketer said on Tuesday. But its share of users, currently at 20 percent, will drop over the years and give Twitter the opportunity to grow its advertising revenue from outside the U.S.