While mobile advertising is growing strongly, apparently most advertisers are sticking with the familiar names they know from the web rather than the new start-ups targeting mobile. “Advertisers are buying up space on TWC, ESPN and other high-profile wireless Web sites, but lesser-known sites are still awash in excess inventory—even if those sites are wildly popular among young mobile users. AirG, for instance, recently surpassed the 20 million-customer mark, delivering its social networking offering in 10 languages and across more than 100 mobile operators. But despite the attractive demographics of its user base, the Vancouver, British Columbia-based startup is struggling to sell more than 10 percent of its ad space…“The last year’s been good,” said AirG spokeswoman Allison Johnson. “We’re definitely getting a lot more interest, but we’re still not selling anywhere near all of our inventory” reports RCR News. The general attitude seems to be that as the industry matures advertisers will be able to determine which mobile sites are the best for them to advertise with, and mobile marketing startups are keen to be there to close the gap. That being said, even AOL (NYSE: TWX) Mobile is reportedly having trouble, with the majority of the inventory left unsold.

As RCR notes, the potential size of the mobile advertising market varies widely depending on who you talk to. However, I’m calling out the line “AdMob said last week it delivered 673 billion—yes, with a ‘B’—ad impressions to U.S. mobile users in September alone.” I’m not exactly sure what they’re referencing, but this PDF of Admob Network Metrics puts the figure at 673 million, with an ‘m’. I’m going with the smaller figure, since 673 billion would roughly require every single person in the US to view an average of 75 mobile ads every day.