As Facebook tours the country with its pitch to investors for the social network’s initial public offering, at least one hole in the company’s plan for the future that has come up repeatedly: What is Facebook going to do about mobile?

The question seems logical, particularly since comScore data released Monday shows that users spend far more time with the mobile version of Facebook’s network than they do on the browser version.

The company had identified mobile as a concern in its initial public offer filings, saying that it would have to adjust its plans for generating revenue as users shifted from the desktop version to the mobile Web and apps.

Since originally filing its paperwork, Facebook has started to target mobile users through the use of “sponsored stories” in the mobile news feed, or posts that come from companies to advertise their products. That’s one of the key ways that the company expects to drive mobile revenue, Facebook’s chief operating officer Sheryl Sandberg said in the company’s online roadshow video.

According to the Facebook-centric blog Inside Facebook, the company is playing around with the mobile ads and asking some users in the U.S. how to make them more effective. The report says some users have seen surveys on their Facebook apps asking how’d they would like to see the ads on the mobile site.

The report says that users are shown two ads from different companies or in different formats and asked which they prefer. Users are given three choices: “neither,” “top ad” or “bottom ad.” The survey can tell Facebook if users like ads with pictures, surveys or other elements as well as what kinds of ads they like to see.

Related stories:

Facebook IPO roadshow in full swing

Achenblog: Facebook IPO show: Why I’m not buying

Facebook users can add organ donor status