Apple’s iAd in-app advertising service is on track to snag 21 percent of the mobile device ad market by the end of 2010. iAd’s growth will come at the expense of Google’s own AdMob, which is expected to dip down to a 21 percent marketshare from 27 percent, according to BusinessWeek.

iAd is Apple’s in-app ad system for iOS-based titles. Apple has been pushing the service as a new approach to drawing in potential customers thanks to its interactive features designed to keep consumers engaged.

iAd marketshare is gaining on Google’s AdMob

As of June, advertisers had committed some US$60 million to iAd advertisements. Companies such as DirecTV, Best Buy and Unilever are on board, and advertisements have been appearing in apps on the iPhone, iPod touch and iPad since July.

While Apple may be grabbing a growing percentage of the mobile advertising market, at least some its competitors aren’t hurting yet. Google, for example, claims that it is still drawing in new clients all the time for its AdMob service.

“If we are losing share, this market is growing faster than any one we’ve seen,” said Jason Spero, Google’s mobile director.

Smaller players in the mobile ad market will see drops in their marketshare, too, according to projections from the market research firm IDC. Microsoft, for example is expected to drop from a 10 percent marketshare down to 7 percent, and Yahoo! will likely dip down to 9 percent from last year’s 12 percent marketshare.