The day after Twitter filed long-awaited plans for its initial public offering of stock, advertisers and industry experts Friday were zeroing in on details revealed for the first time about the microblogging service's sprawling user base.

Perhaps one of the more intriguing nuggets in the filing with the Security and Exchange Commission is that a full 75 percent of Twitter's 200 million active users access the service on a mobile device every month.

With the mobile advertising market heating up, Twitter is taking full advantage of that user base. The company reported in its stock prospectus that 65 percent of its total advertising revenue -- the lion's share of the $254 million Twitter took in during the first half of the year -- was reaped from mobile devices.

That's much higher than the 41 percent rival Facebook claimed in its most recent quarterly report. Even that number, though, was enough to send Facebook's share price skyrocketing after months in the doldrums. That's why some experts see Twitter's mobile growth as one of its biggest potential selling points on Wall Street.

"Unlike Facebook, Twitter was birthed on mobile. That's a huge differentiator," said Marcus Nelson, chief executive of a San Francisco startup called Addvocate that helps companies better promote themselves on social media.

Both social networks generate the bulk of their revenue from advertisements. On Facebook, that's in the form of "suggested" posts inserted in a user's message timeline, along with more traditional display advertising off to the side of the screen.

Twitter, meanwhile, lets businesses pay to insert "promoted tweets" in the streams of 140-character messages that make up the user experience. Nelson, for one, calls it a superior approach.

"Twitter has a revenue model that works," he said, noting that its users digest more than a billion messages -- promoted or not -- each week. "Users have accepted a simplified ad platform that also happens to deliver results to advertisers."

Twitter CEO Dick Costolo has pushed the gospel of promoted tweets since taking the company's reins in late 2010. Twitter's ad revenue that year was just $7 million.

And some experts say the IPO prospectus shows a clear road ahead to even bigger revenue growth. That's because while the vast majority of Twitter's users are overseas, just 25 percent of its revenue came from abroad.

The company's most recent data shows that 169 million people outside the United States used the service at least once a month -- more than three times its U.S. user base. Twitter's IPO filing calls international ad sales a target for future revenue growth.

"Their sales teams are focused on the U.S. and other developed markets where they get more ad dollars -- the low-hanging fruit," said Tom Bedecarre, chairman of San Francisco digital marketing agency AKQA. His company has helped big brands such as Wheat Thins promote themselves on Twitter.

Bedecarre said the numbers revealed in the filing demonstrate Twitter's potential appeal to marketers no matter where they're targeting customers.

"Twitter delivers a very big U.S. audience of 50 million people," he said. Meanwhile, "Other global marketers who sell products and services around the world will be very interested in the global scale of 218 million monthly active users."

But Morningstar analyst Rick Summer, in a note to clients, warned that while there were encouraging signs of growth in the prospectus, would-be investors should proceed with caution.

"Other social networking companies on our coverage list range from modestly overvalued (Facebook) to inexplicably overvalued (LinkedIn)," Summer wrote. "We believe Twitter's IPO pricing may reflect a similar level of optimism."

He also said that while those rival companies have built "economic moats" around their user bases -- a person's business contacts in LinkedIn's case, or his friends and family in Facebook's -- "Twitter's network is more akin to a highly personalized media platform," where connections among users may be more fleeting.

"Although Justin Bieber is the most followed Twitter user today, we think it is highly unlikely that he will have the same level of engagement in 10 years," Summer wrote. That lack of user loyalty to the network makes it harder to predict future revenue growth, he said.

While Summer has yet to assign a formal rating to Twitter's stock, he noted that $448 million in revenue over the past four quarters -- and a growth rate that's outstripped Facebook's and LinkedIn's over that time -- make the company an undeniable heavyweight in the social networking world.

But Twitter has yet to make a profit, in part because of its heavy spending on new technology and employees. And given that social networking is still a nascent market, Summer wrote, "there is a great deal of uncertainty" about how profitable Twitter ultimately can be.

Staff writer Jeremy C. Owens contributed to this report. Contact Peter Delevett at 408-271-3638. Follow him at Twitter.com/mercwiretap.