Because the app has been out since 2008, gone through multiple iterations, currently holds a 2-star rating on the App Store, and still hasn't managed to monetize 60% of their impressions.
So what you mean to say is, we have here a company whose traffic has been bleeding off their monetized, desktop platform into the unmonetized, mobile platform that has existed for four years, and they still haven't done anything about it?
Are we still talking about FB? I swear this sounds more like RIM.
Right, so Facebook could potentially grow by 150%, maybe. But their valuation is predicated on them growing by a factor of 10x or more. That's the issue.
Now take the 9 million sites with the Like button and throw in some extra Javascript for a 'Social Adsense' product. Now they have implicit and explicit data to target off of. There's your 10x growth in revenue.