Limbo has announced that 500,000 new members have joined the mobile community since the July 11 launch of its social location tools and iPhone application.

The iPhone application spurred adoption by subscribers of all carriers who access the community from other platforms such as the Web, mobile Web and text messaging. The growth rate is the greatest in the company's three-year history.

"The iPhone application itself has been successful, but we were also surprised by the growth it has created in the bigger universe," said Jonathan Linner, CEO of Limbo, Burlingame, CA.

"Only a small percentage of the population has an iPhone, but the viral spread of people using the iPhone app in a community setting has caused them to tell their friends who don't have an iPhone, so it's been a catalyst for growth in other areas as well," he said.

Consumers can access the application via SMS and the mobile Web at http://www.limbo.com, http://m.limbo.com or http://phone.limbo.com.

"The community can only exist if everyone can access it, so we made sure you can access Limbo via any device and that's a big part of what we're excited about," Mr. Linner said.

"It's a social tool, and there's a fabulous user experience for the iPhone, which is a top-end product, buta if you have an iPhone but your friends don't, you can communicate with them on whatever device," he said.

Approximately 25 percent of the 500,000 new members are iPhone users.

Increased activity has been seen across all community tools and has significantly increased the number of community interactions per member.

Limbo members generated more than 57 million messages in July, up 22 percent from 44 million in June.

Last month, Limbo launched a series of mobile-location tools and a corresponding iPhone application that lets members connect with their friends by showing where they are and what they're doing.

In addition to new member growth, consumers who previously used Limbo exclusively for its text-message alerts or social games are now rapidly adopting the mobile-location services.

Limbo's SMS alerts deliver everything from jokes to weather updates.

Limbo has more than 2.5 million members and a monthly reach of more than 5 million people.

"The trend of things is looking great, since it took a while to get to a million, but we got to 2 million pretty quickly and we've already surpassed 2.5 million," Mr. Linner said. "We're seeing pretty fast momentum now,and 2009 is going to be a big breakout year for mobile."

The privately held company is backed by three venture capital funds: Azure Capital Partners, Draper Fisher Jurvetson and New Enterprises Associates.

Limbo is free for everyone to use and is paid for by mobile advertising.

"We're long-time advocates of mobile advocates since our Enpocket days, and we believe mobile content should be free to consumer," Mr. Linner said. "We don't believe in copying other formats, copying Web models such as banner ads and forcing them into mobile, so we want to merge advertising into the content whenever possible."

Large brands that have advertised on Limbo's mobile platform include Sega and Nokia.

Fast-food chain Jack in the Box ran a mobile marketing campaign on Limbo promoting a new line of smoothies. Mobile coupons offered two-for-one and $1-off discounts.

"We were able to target people within two miles of a Jack in the Box location and drive them in to redeem the mobile coupons," Mr. Linner said. "Mobile is a uniquely powerful platform in that way and the future of mobile is surrounded by advertising."

LBS is a hot topic throughout the mobile industry and Limbo is no exception.

"Location-based services are an interesting development in mobile advertising, now that we can know where you are and what you're doing," Mr. Linner said. "It helps us figure out the things you want to see promoted and target by location, which starts to unlock the power of mobile advertising."

Limbo runs weekly promotions with Verizon, Alltel and Virgin Mobile. These carriers promote Limbo on-deck.

"We do a variety of things to spread the word about Limbo, and the viral aspect is key," Mr. Linner said. "Peer-to-peer recommendations are far more powerful and significant than anything we can tell them, although we do have deep relationships with carriers."

Limbo executives have expressed optimism that mobile marketing is about to experience an explosion.

"Generally we're pretty excited about what we see going on, lots of brands adopting mobile initiatives and people doing mobile are spending more quarter to quarter and year to year," Mr. Linner said.

"Brands' initial entry into mobile is usually successful and companies that keep using it are getting good strong results, better and better over time," he said. "Because it's a relatively new platform people not investing in innovative formats more relevant to the mobile medium, instead going with simple banners on a WAP page.

"Mobile marketing is more exciting and effective when it's based on context, with location-targeted and action-targeted deployment of advertising."