No doubt people are downloading more apps than ever from the iPhone store.

But as the number of downloads soars past 60 million, the percentage of those that are used daily continues to slide.

That's according to Greg Yardley, CEO of PinchMedia, a New York startup which provides ad softawe and analytics tools for iPhone developers.

Whereas 20% of the Pinch Media-equipped apps downloaded from the Apps Store were being used daily four weeks ago, that number has slid to 15% and will likely continue to drop, Yardley says.

The company has 30 apps using its tools, which Yardley says have been installed on about 750,000 of the devices, including the original iPhone and the new 3G model.

While Apple is now releasing daily download figures so developers can track their apps, it's not releasing daily usage figures.

That's where PinchMedia, which we first told you about last week, comes in. 

PinchMedia, which has partnered with KickApps, is one of several startups betting that the Apps Store ecosystem will be able to nurture and sustain businesses for mobile ad companies. Older startups like AdMob are making plays also.

Given the daily usage figures, you can make an argument that people are getting bored  with the majority of iPhone apps.

When most of the apps are free, it's easy for consumers' eyes to be bigger than their attention spans when browsing the Apps Store. Free apps that look cool at first glance but ended up gathering dust will no doubt always clog up people's iPhones.

But the dropping rate of daily usage could have another explanation, says Yardley.

"There's lots of choices out there now. We can expect that number to come down," says Yardley, formerly with RightMedia, now part of Yahoo.

Given that less than one in five apps Yardley is tracking see daily use, iPhone users may start gravitating toward paid apps that offer a free trial period, which very few now provide.

When asked if developers are worried about the low usage rates, Yardley said most are concerned about the length of time it takes to get an app approved for the store.

"That's people's primary concern --  although the process is getting better."

The concern is understandable, because getting first mover advantage on a platform is critical, as Fred Wilson noted in this post when talking about Slide, RockYou and other shops that now dominate the Facebook platform. 

My guess is that iPhone platform will follow a similar trajectory, with a handful of entertaining or useful apps dominating the platform while the vast majority get little traction.