There is concern among investors that Google is generating too many mobile ad clicks, but not enough revenue from them. Google will release its earnings report this week, and it seems Wall Street isn’t sure how much improvement to expect in that department.

Google CFO Patrick Pichette said in an earnings call in January (transcript via SeekingAlpha), “Our aggregate cost of click growth was down 8% year-over-year and quarter-over-quarter. Remember, too, that this is an aggregate number, which includes both google.com and our AdSense properties. On this, it’s important to look at CPCs and clicks together.”

Mobile costs per click are often lower than desktop CPCs, which is what has investors and analysts worried.

The Wall Street Journal brings up the topic ahead of this week’s earnings release, and mentions a Marin Software report, which showed the percentage of clicks Google gets from mobile search ads will grow to 25%, with prices remaining lower than their desktop counterparts. “Increased demand is a good thing for Google, but it also means the company must make up for declining prices with more volume,” the Journal’s John Letzing assesses.

It’s clear that Google is passionate about mobile. Google CEO Larry Page put out a letter to investors last week, in which he described Google’s focus and sense of direction. He talked about a lot of things, including mobile.

“Android is on fire, and the pace of mobile innovation has never been greater,” Page said in the letter. “Over 850,000 devices are activated daily through a network of 55 manufacturers and more than 300 carriers. Android is a tremendous example of the power of partnership, and it just gets better with each version. The latest update, Ice Cream Sandwich, has a beautiful interface that adapts to the form of the device. Whether it’s on a phone or tablet, the software works seamlessly.”

Meanwhile, Google is pushing heavily for sites to “Go Mo” (mo means mobile).

Later in the letter, Page said, “We understand the need to balance our short- and longer-term needs because our revenue is the engine that funds all our innovation. But over time, our emerging high-usage products will likely generate significant new revenue streams for Google as well as for our partners, just as search does today. For example, we’re seeing a hugely positive revenue impact from mobile advertising, which grew to a run rate of over $2.5 billion by the third quarter of 2011—two and a half times more than at the same point in 2010. Our goal is long-term growth in revenue and absolute profit—so we invest aggressively in future innovation while tightly managing our short-term costs.”

That explains that right?

Still, it doesn’t mean the quarter will necessarily be where investors want it to be. We’ll find out on Thursday.

Increasing mobile device usage doesn’t only concern Google. Rival Facebook, for example, cited mobile use among its key risk factors in an IPO filing earlier this year. With the IPO expected in May (on the NASDAQ), Facebook has been doing plenty of its own work trying to create a better mobile experience for its users.

Clearly, Google has a significant lead in terms of mobile integrations (and ads) in mobile, over Facebook, simply by having Android and Motorola Mobility. And if these two companies compete on more and more fronts, as many think they are likely to do, strong mobile showings are going to be imperative.