• by Mark Walsh  @markfwal, December 7, 2009

Facebook has crossed the 100-million-user mark in the U.S. a week after announcing it had surpassed 350 million members worldwide. That's a nice round audience figure to present to advertisers in the market representing the bulk of its revenue.

The social network today hit 100.5 million U.S. users Monday -- up from 98.1 million a week ago, and has averaged a few million new domestic users over most of the last year, according to Inside Facebook. That makes the U.S. the first country to reach the 100 million threshold, although 70% of Facebook's monthly active users live elsewhere.

Last month, comScore estimated Facebook's U.S. audience in October at 97.4 million unique visitors -- well ahead of the 82.9 million for Fox Interactive Media, whose chief property is MySpace. Facebook eclipsed its social networking rival in U.S. traffic in May, according to comScore.

Facebook is rumored to reach revenue of more than $500 million this year, and in September, founder and CEO Mark Zuckerberg said the company was generating positive cash flow ahead of schedule. Facebook will grow to $710 million in revenue in 2010, according to an estimate from advisory firm and market-maker NYPPEX cited in a Wall Street Journal report yesterday.

Building its audience in the world's biggest advertising market will only help Facebook to reach that level. The site's U.S. demographics are also attractive to marketers, with 52% of users between 18 and 34 years old and 19% from 35 to 44.

Brands that maintain a Facebook presence also stand to gain through Google's move to begin including status updates, video and other content from Facebook Pages in newly launched real-time search results. The step could help increase traffic to brand pages.