By Gerry Shih

SAN FRANCISCO (Reuters) - Twitter Inc (TWTR.N) on Tuesday reported 255 million monthly active users, up 5.8 percent from the previous quarter but not enough to satisfy investors increasingly concerned about its struggle to gain a mass following.

Twitter's stock fell nearly 10 percent after hours, to below its post-initial public offering low of $38.80 on Nov. 25.

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 fewer than some analysts' estimates.

The results revealed slowing momentum at a company that exuberant investors had once argued could match Facebook Inc's (FB.O) scale. At its peak in December, Twitter enjoyed a $46 billion(27.33 billion pounds) market capitalization on just $665 million of revenue in 2013, making it one of the world's priciest stocks.

But cracks began to show in February, when Twitter Chief Executive Dick Costolo pledged to revive user growth after disclosing the rate had fallen to its lowest in years. Since then, Twitter's stock has been trading at 40 percent below its peak.

Expectations of Twitter growing into a communications utility that Facebook has become are "unrealistic and divorced from reality," said Brian Wieser, an analyst at Pivotal Research. "Twitter is and will remain a niche medium, and a very powerful one."

The company, which has been steadily refining its targeting capabilities, shows signs that it is better able to present ads based on what it thinks each user would be interested in.

Twitter said its advertising revenue per thousand timeline views, a metric that measures the effectiveness of its ads, nearly doubled to $1.44.

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

Excluding certain items, Twitter broke even against Wall Street expectations of a 3 cent per share loss. But the company said its net loss in absolute terms widened nearly fivefold to $132 million from $27 million a year ago.









(Reporting by Gerry Shih; Editing by David Gregorio and Richard Chang)