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.