In the long term, a P/E of 15-25 is the more 'normal' range. With a 90 P/E, Facebook has to quadruple its earnings to get to normal. It this possible? Yes. Likely? I don't know.
I am not a stock analyst, but I love numbers and try to get to logical conclusions. I've seen data that worldwide advertising is about $400B, and US about $100B. If Facebook's profit runs 25% or so and I want a P/E of 20, it needs profit of $5B on sales of $20B (to reconcile its current $100B market cap). No matter what FB growth in sales is, the advertising spent worldwide will not rise or fall by much more than the economy. So with a focus on ads, they would need about 5% of the world market to grow into a comfortable P/E.
Flipping this around, if all advertising were 25% profit (a crazy assumption), there are $100B in profit to be had world wide each year, and the value of the companies might total $2T in aggregate.
The above is a rambling sharing of the reasonable bounds one might expect in analyzing a stock. It can be used for any otherwise finite market, such as soft drinks. There are only so many people on the planet, and in aggregate, the total soft drink consumption can't exceed, say 6 billion gallons per day. The pie may grow a bit, but it's considered fixed as an order of magnitude.
Edit - for what it's worth, as of 8/3/12, the price has dropped significantly, currently $20, and the P/E is showing as 70X. I'm not making any predictions, but the stock needs a combined higher earnings or lower valuation to still approach 'normal.'
You could try this experiment:
pay for an Ad/banner on Facebook for 1 month. The Ad/banner should link to your ecommerce site.
Then see if the Ad/banner does or does not convert into ecommerce orders ("converting" means that people coming to your eccomerce site from Facebook after having clicked on your Ad/banner really buy something on your site).
If it does convert, you will go on paying for Ads/banners and other people will do the same for their sites, so FB might make cash in next years. But if it does NOT convert you and everybody else will soon discover and stop paying for Ads/banners, thus it will be hard for Facebook to make money with Advertising, thus Facebook might be just a big bubble (unless they find other ways of making money).
I did the experiment I suggested above and the conversion rate was an absoulte ZERO!!! (Instead Google Adwords converted well for the same site). So IMHO I would stay away from FB.
But remember that stock market is emotional (at least on short periods of time), so it might be that even if FB wil never become a cash cow, for the 1st few months people (expecially small investors tempeted by the brand) might go crazy for the stocks and buy buy buy, making the price go up up up.
EDIT in reply to some comments below arguing that my answer was boiled down to one single experiment:
General Motors said Tuesday that it will stop paid advertising on Facebook...the social media paid ads simply weren't delivering the hoped-for buyers... (CNN May/15/2012)
A donkey can not fly either when it's me (with a single experiment) trying to make it fly or the entire GM workforce.