After quietly entering the mobile advertising market, Facebook in-app advertising service LifeStreet Media is now looking to become a major player in the industry. The company announced today it has raised $66 million in new funding from Nautic Ventures in order to help it continue its mobile expansion plans.

Founded in 2005, the San Carlos-based company is one of the biggest in-app advertising services on Facebook, counting Zynga, Big Fish Games and Classmates among its clients. The company and now reaches over 350 million unique users every month on Facebook, iOS and Android. According to LifeStreet’s CEO and co-founder Mitchell Weisman, the mobile portion of those users is growing rapidly.

Although already successful on Facebook, LifeStreet faces some stiff competition in the mobile advertising space. There are established players like Google’s AdMob, Apple’s iAd, ValueClick owned Greystripe and the newly public Millennial Media to contend with. There are also well-funded independents like InMobi, which is backed with $200 million from Japanese conglomerate Softbank, and JumpTap which has raised $94 million in total. The company will also been going head-to-head against more niche service companies like PlayHaven and Chartboost.

Competitors aside, Weisman believes LifeStreet will be able to differentiate itself from other mobile ad companies with its proprietary RevJet platform. The company has spent $25 million of its own profits developing the technology, which allows advertisers to test multiple variables like copy, design, colors and performance data simultaneously. The service has already been successful for LifeStreet’s Facebook advertising service and Weisman is confident it will see the same results in mobile.

“From our perspective, developers who create apps for mobile are the same guys, and in some cases, literally the same guys who are developing apps on Facebook. They have the same challenge,” he explains.

According to Weisman, RevJet allows developers and advertisers to quickly and easily find the specific combination of performance variables in their display ads that will translate into the highest conversion and revenue rates possible, an approach that is as relevant to Facebook apps as it is to mobile apps.

LifeStreet currently has 80 employees working in California, Russia, Latvia and the Ukraine. It had previously raised a $1.8 million Series A round, lead by Exigent Ventures and a $1.8 million Series B round from Angel investor Archie Cox. LifeStreet will be using the $66 million investment from Nautic Ventures to expand its mobile and social advertising business and to provide liquidity to earlier investors.