UPDATE 2-Twitter shares tumble as flat user growth disappoint investors

(Adds comments from COO, analyst quote, share prices)

July 27 (Reuters) - Twitter Inc shares tumbled more than 10 percent in premarket trading on Thursday after the social media platform disappointed investors with stagnant monthly active user growth in the second quarter.

Despite its appeal among celebrities and public figures, Twitter has struggled to sustain its closely watched user growth even as it invested in features and live content to help draw viewers and boost user engagement.

It is in stiff competition for advertising dollars with other platforms like larger rival Facebook Inc and Snap Incs messaging app Snapchat.

Twitter had 328 million average monthly active users (MAU) in the three months through June 30, unchanged from the previous quarter. Analysts were expecting 328.8 million, according to financial data and analytics firm FactSet.

Twitter shares had run up some 40 percent since mid-April as investors bet on another quarter of growth after the microblogging service reported that it added 9 million more monthly active users than expected in the first quarter.

"If you really can't accelerate MAU interest given the daily tweets from POTUS, not sure when you will," said Michael Nathanson, senior research analyst of MoffettNathanson Research, referring to U.S. President Donald Trump's frequent use of Twitter.

Trump, one of the most active politicians on Twitter, has tweeted multiple times a day on average since his inauguration in January, according to social media analytics company Zoomph.

"The positive contributions to MAU growth from product improvements in the second quarter were offset by lower seasonal benefits and other factors, resulting in flat MAU quarter-over-quarter," said Chief Operating Officer Anthony Noto during a conference call with analysts.

In the United States, Twitter's average monthly active users declined to 68 million in the second quarter from 70 million in the first quarter.

Monthly active users, a key performance indicator for social networking services, is typically calculated by tabulating the number of users who have logged in and logged out during the 30-day period.

Twitter's second-quarter net loss widened as it took a $55 million impairment charge related to an investment writedown and revenue fell 4.7 percent.

The company has been trying to boost revenue through livestreaming deals, but had a setback in April when it lost a deal to livestream NFL games this year to Amazon.com Inc .

Advertising revenue fell 8 percent to $489 million, but well exceeded the $458.1 million estimate.

The company's net loss widened to $116.5 million, or 16 cents per share, in the second quarter ended June 30, from $107.2 million, or 15 cents per share, a year earlier.

Excluding items, the company earned 8 cents per share.

Revenue fell about 4.7 percent to $573.9 million, the second time it has fallen since Twitter's debut in 2013.

Shares of Twitter fell more than 10 percent to $17.63 in premarket trading on Thursday. (Reporting by Angela Moon in New York, Pushkala A and Aishwarya Venugopal in Bengaluru; Editing by Bernadette Baum and Meredith Mazzilli)