Twitter Inc's shares fell more than 11 percent in extended trade on Tuesday after the microblogging company said its number of monthly average users grew at the slowest pace since it went public in 2013.
"This is unacceptable and we're not happy about it," Jack Dorsey, who stepped in as interim chief executive on July 1, said on a call with analysts.
Twitter said it had 304 million core users in the second quarter, up from 302 million in the prior quarter.
Twitter's struggles to increase its audience worries investors, who are focused on the company's growth potential, and the latest figures did little to reassure them.