Three Digit PE! / How much bigger can Facebook get?

Apparently, Facebook's IPO will put it in the fabled faery lands of companies with three digit P/E ratios. Specifically, it looks like Facebook may well have a P/E of 100 when it goes public. For reference, Apple's P/E is currently 13.6 and Google's is 18.55. If Apple's P/E was 100, it's market cap would roughly be 3.8 TRILLION dollars, and similarly, Google would be valued at 1.07T. Granted, P/Es that high are not unheard of, but, typically, they are very big when a company is still very early in its growth curve, or, on the way down, when earnings are particularly low but there is still fundamental underlying value to the company. For instance, EA currently has a P/E of ~66. This honestly is probably not great for EA, and, in fact, earnings are way, way down for the gaming industry as a whole right now, apparently to the tune of 25% March 2011 v. March 2012, and 32% April 2011 to April 2012.

So, no matter how you look at it, Facebook's valuation is really, really high. Given it currently has 900,000,000 users, and given that 80% of the world's 7B residents live on less than ~$3600 a year, it looks like the total addressable market for facebook is 1.4B... meaning that, globally, there are only around 500M potential users left. Accordingly Facebook can only realistically grow another 15.5%, if one just assumes linear extrapolation and ignores a whole lot of other details. I think that, in broad strokes, however, this rough estimate is actually quite telling. Facebook is so huge it already probably has the majority of its global addressable market. So unless it can start to figure out how to get a whole lot more money per user, they cannot rely on additional growth to justify their P/E, because they are approaching total market saturation. Update: Apparently, GM has just stopped buying paid ads on Facebook. According to the WSJ:

General Motors Co. plans to stop advertising on Facebook after the company's marketing executives determined their paid ads had little impact on consumers, people familiar with the matter said, a move that comes as more companies question the effectiveness of advertising on the social networking site.

I'm still betting on a first day bump for Facebook, but, hey, I may be wrong.

Tl;dr Facebook's valuation is 100x its yearly earnings, which is unsustainable given that they don't have much more room to grow, and that multiple is way, way higher than other tech giants.

Edit/update: A friend of mine pointed out that a mere $9 profit a year, per user, for Facebook, would generate 8.1B in yearly earnings. This would put their multiple at 12.34, which is actually pretty solid. So, that raises the following questions about ceilings:

1. If Facebook has 900M users, and is generating ~1B in year profits, is it possible to get to 8B in profits? To put it another way, is the way in which Facebook users interact with the site worth $9 per year, above costs, as opposed to the current $1 per year?
2. Of those 900M users, how many are actually active accounts? More specifically, is it already possible that Facebook has already reached a maxima of user-hours?

To reiterate, that's two questions: the first is about the value of user hours, the second is about whether Facebook can increase the number of user hours. Only time will tell.