In the sixth update to its S-1 filing so far, Facebook admitted in unambiguous terms on Wednesday that it's in trouble on the mobile front and not quite sure what to do about it. Trouble doesn't mean that they're lacking mobile users -- in fact, 488 million people used Facebook on a mobile device in March alone. The challenge, rather, is generating meaningful revenue from all that activity. This is a unwelcome chunk of bad news coming from the company set to make its initial public offering in just eight days. 

We'll bold the key snippet from the updated S-1 filing for clarity. Note the use of the phrases "meaningful revenue" and "unproven," emphasis ours:

We do not currently directly generate any meaningful revenue from the use of Facebook mobile products, and our ability to do so successfully is unproven. We believe this increased usage of Facebook on mobile devices has contributed to the recent trend of our daily active users (DAUs) increasing more rapidly than the increase in the number of ads delivered. 

So why is this a bad thing? The S-1 immediately explains, emphasis ours again:

If users increasingly access Facebook mobile products as a substitute for access through personal computers, and if we are unable to successfully implement monetization strategies for our mobile users, or if we incur excessive expenses in this effort, our financial performance and ability to grow revenue would be negatively affected.”

You have to give Facebook some credit for being so candid. (Then again, federal regulations around IPOs require such candor.) They're not wrong to flag the importance of mobile, a space that's expected to generate $1.8 billion in advertising this year. It was also recently revealed that people now spend more time gazing at content on their mobile devices than they do looking at print. It's also worth pointing out, however, that Facebook users still spend the vast majority of their time clicking through the desktop version according to the latest comScore data. Unfortunately, Facebook's revenue as a whole was down six percent in the first quarter of this year, a signal that their advertising business is slumping in general.

It's not hopeless. And trust us: Believe us, Facebook will be fine. Next week's IPO is expected to value the company at up to $96 billion, and as the overcrowded, ultra-hyped roadshow events made clear, there's no lack of excitement about what looks like it will be the biggest IPO this country's ever seen. Meanwhile, Facebook is making moves to open new frontiers on its business. Around the same time that they amended their S-1 filing with the bad mobile news, Facebook launched an App Center, an equivalent to Apple's App Store and Google Play. With a strong emphasis on mobile, the App Center is filled with everything from games like Draw Something to music apps like Spotify to photography tools like Viddy. And of course, you're allowed to pay both for apps and in-app purchases. It's not necessarily a silver bullet, but it's something.

We're sure Facebook has some more mobile money-making ideas up its sleeve. Many say that Facebook bought Instagram for $1 billion to beef up its presence in mobile. Another idea that would definitely throw some armor over Facebook's mobile weak spot is an ambitious one: The Facebook phone. While we're pretty sure Facebook isn't going to get into the hardware business, they do seem poised to launch a mobile operating system, which would give them endless real estate to serve ads up to their addicted users. Then again, they could also just start serving more ads on their existing mobile ads. But they won't because Mark Zuckerberg doesn't want too many ads on Facebook, and as we all know too well, Mark Zuckerberg is the king of Facebook. His word is law.