Web traffic to micro-blogging service Twitter Inc. has flat-lined in recent months, according to measurement group comScore, a discordant note for the startup that only last month secured new funding valuing the company at $1 billion.

The number of U.S. visitors to Twitter.com remained at about 21 million in September, the third month that traffic has stayed at about the same level. The data comes as Twitter faces increasing pressure to demonstrate it has the staying power and business model needed to generate revenue.

ComScore's traffic data doesn't take into account users who access Twitter through third-party applications and Web sites such as TweetDeck, Tweetie and Seesmic, or through their mobile phones. But the data suggest that Twitter's torrid growth - from 1.2 million visitors in September 2008 to 20.1 million visitors in June 2009 - appears to have stalled, at least for the time being.

Twitter did not respond to a request for comment.

ComScore also noted the average number of visits per user, as well as the amount of time each visitor spends on Twitter, remained roughly stable over the past three months.

Twitter's international traffic has continued to grow, increasing from 51.6 million in July to 54.7 million in August, but growth was down sharply from earlier this year when monthly growth rates surpassed 50%. ComScore is due to release Twitter.com's international traffic numbers for September later this week.

The San Francisco-based micro-blogging service, which lets users blast short messages their from computers and mobile phones, has been one of the hottest brands in tech over the past year or so.

The company, which has not yet explained how it will generate revenue, has attracted interest from Internet giants like Google Inc. (GOOG) and Microsoft Corp. (MSFT). The two are believed to be pitching Twitter on possible partnerships to help it sell advertising.

Twitter last month secured about $100 million from a group of investors that includes mutual-fund giant T. Rowe Price Group Inc., private-equity firm Insight Venture Partners, and venture group Benchmark Capital.

-By Scott Morrison, Dow Jones Newswires; 415-765-6118; scott.morrison@dowjones.com