Its shares fell 8.5 percent to USD 39 in extended trading.

The San Francisco-based company also said viewers refreshed their "timelines" - Twitter's equivalent of Web pageviews - 157 billion times in the first quarter, slightly below what some analysts had hoped for.

The company, however, posted better-than-expected quarterly revenue of $250 million, as its mobile ads drew more viewer responses.

Twitter Chief Executive Dick Costolo pledged in February to revive user growth after disclosing it had fallen to its lowest rate in years. Its shares have been trading 40 percent below their peak as investors wait to see whether the service could broaden its appeal.