Are you planning to run ads on Facebook that would be seen on the mobile phones of its millions of users? In contrast to some early indications that engagement on these ads is high, one company that specializes in visitor eye tracking has released a report claiming that Facebook’s mobile ads in its iPhone app are seen by a lower percentage of users than those on the web or tablets.

Only 3 – 13% of users in the study were impressed upon by below-the-fold ads (ads far enough down the page or feed that your would-be customer has to scroll down to see them) on mobile, but the fixed position of ads on the web made impressions more healthy and predictable.

Note that the good news we’ve seen in the past about Facebook’s mobile-targeted Sponsored Stories is that once a Facebook visitor has actually tapped through a mobile ad to view your Page, he or she is 63% more likely to Like your Page than a visitor who arrived through a desktop web ad. This new study by EyeTrackShop addresses the frequency and length of the impression, not the engagement after the impression has been made.

Given the similarity between the iPhone and mobile web experiences, it makes sense to extend the lessons of these findings to the experience on other smartphone platforms like Android, too. Interestingly, iPad ads didn’t just perform better in EyeTrackShop’s study than iPhone ones; the study concludes that tablets present the best opportunity for an impression from any platform.

As we noted last week, Facebook’s difficulty in gaining dramatic traction with its IPO may be attributed partially to the company’s shaky mobile strategy. About half of people with Facebook accounts use the social network on their mobile devices, according to data cited in EyeTrackShop’s study — that’s more than 400 million people.

You can flip through the pages of EyeTrackShop’s presentation yourself at CNET. EyeTrackShop is the same company that produced a similarly helpful study about which parts of a Facebook Page visitors are most likely to examine carefully.

[Image credit: Casey Fleser]