Quote from Ghost of Cutten:
You cannot compare a dominant retail business doing 40bn in sales, with a market cap of 80bn, with a startup social network doing 500m in sales, and a market cap of 9bn. The scope for growth in the latter is clearly much greater, merely due to its small size. The economics are totally different, the outlook is different, the valuation is different - it is frivolous to compare them.
Also, growth stocks aren't valued on historic earnings, they are valued on future earnings estimates - as in, what will they be earning in 5 or 10 years time. So forget today's PE because it is irrelevant, what matters is the 2020 EPS and whether today's price under or overestimates the likelihood of getting there. A growth stock might have negative earnings for 2012, but be making 5 billion a year by 2020. Whereas a 'cheap' value stock might be making 1 billion this year, and be losing money 5 years in a row by 2020. Historic earnings are almost irrelevant for businesses undergoing rapid change (whether positive or negative).
Take a look at some past growth stocks e.g. YHOO, MSFT, CSCO, DELL etc (or even earlier AAPL) during their growth phases and look at what PE they sold at and how their EPS grew, and what that all meant for the valuation of the stock.